The problem is you have a real weak computer.....see the dxdiag below. 
Start out by first making sure your computer is "clean and mean". Defrag it, then get your processes down to under 30 (hit control alt delete, click the processes tab and see how many you have running in the lower left corner of that window) 
Alacrity is a good program to use to get them turned off.
Once you get your computer lean and mean, you have to set the game up so it will be as easy on your computer. These are Skuzzy's suggestions on those set-ups.
1)  Make sure the "Maximum Texture Size" is set to 256, no higher, in the "Video Settings".
2)  Make sure the anti-alias slider is set to off (all the way left).
3)  In the game, make sure the only "Options->Graphic Detail->Advanced" option checked is "Bump Map Terrain".
Try it out like that at first. See how it works. If you have no troubles with it like that try turning on a few things to get as much "eye candy" with out it crapping out. Good luck!
------------------
System Information
------------------
Time of this report: 8/10/2009, 16:10:37
       Machine name: YOUR-XHTR8HVC4P
   Operating System: Windows XP Home Edition (5.1, Build 2600) Service Pack 1 (2600.xpsp1.020828-1920)
           Language: English (Regional Setting: English)
System Manufacturer: HP Pavilion 061
       System Model: DM181A-ABA a305w
               BIOS: Version 3.21
          Processor: Intel(R) Celeron(R) CPU 2.70GHz, ~2.7GHz
             Memory: 1526MB RAM
          Page File: 351MB used, 3029MB available
        Windows Dir: C:\WINDOWS
    DirectX Version: DirectX 9.0c (4.09.0000.0904)
DX Setup Parameters: Not found
     DxDiag Version: 5.03.0001.0904 32bit Unicode
------------
DxDiag Notes
------------
  DirectX Files Tab: No problems found.
      Display Tab 1: The file nv4_disp.dll is not digitally signed, which means that it has not been tested by Microsoft's Windows Hardware Quality Labs (WHQL).  You may be able to get a WHQL logo'd driver from the hardware manufacturer.
        Sound Tab 1: No problems found.
          Music Tab: No problems found.
          Input Tab: No problems found.
        Network Tab: No problems found.
--------------------
DirectX Debug Levels
--------------------
Direct3D:    0/4 (n/a)
DirectDraw:  0/4 (retail)
DirectInput: 0/5 (n/a)
DirectMusic: 0/5 (n/a)
DirectPlay:  0/9 (retail)
DirectSound: 0/5 (retail)
DirectShow:  0/6 (retail)
---------------
Display Devices
---------------
        Card name: NVIDIA GeForce FX 5500
     Manufacturer: NVIDIA
        Chip type: GeForce FX 5500
         DAC type: Integrated RAMDAC
       Device Key: Enum\PCI\VEN_10DE&DEV_0326&SUBSYS_00000000&REV_A1
   Display Memory: 256.0 MB
     Current Mode: 1024 x 768 (32 bit) (60Hz)
          Monitor: Plug and Play Monitor
  Monitor Max Res: 1600,1200
      Driver Name: nv4_disp.dll
   Driver Version: 6.14.0010.7776 (English)
      DDI Version: 9 (or higher)
Driver Attributes: Final Retail
 Driver Date/Size: 7/16/2005 17:09:00, 3908608 bytes
      WHQL Logo'd: No
  WHQL Date Stamp: None
              VDD: n/a
         Mini VDD: nv4_mini.sys
    Mini VDD Date: 7/16/2005 17:09:00, 3198208 bytes
Device Identifier: {D7B71E3E-4066-11CF-D07D-0A2000C2CB35}
        Vendor ID: 0x10DE
        Device ID: 0x0326
        SubSys ID: 0x00000000
      Revision ID: 0x00A1
      Revision ID: 0x00A1
      Video Accel: ModeMPEG2_A ModeMPEG2_B ModeMPEG2_C ModeMPEG2_D
 Deinterlace Caps: {212DC722-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(YUY2,YUY2) Frames(Prev/Fwd/Back)=(0,0,2) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {212DC723-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(YUY2,YUY2) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_MedianFilteri ng
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(YUY2,YUY2) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalSt retch
                   {212DC722-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(UYVY,UYVY) Frames(Prev/Fwd/Back)=(0,0,2) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {212DC723-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(UYVY,UYVY) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_MedianFilteri ng
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(UYVY,UYVY) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalSt retch
                   {212DC722-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(YV12,0x32315659) Frames(Prev/Fwd/Back)=(0,0,2) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {212DC723-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(YV12,0x32315659) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_MedianFilteri ng
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(YV12,0x32315659) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalSt retch
                   {212DC722-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(NV12,0x3231564e) Frames(Prev/Fwd/Back)=(0,0,2) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {212DC723-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(NV12,0x3231564e) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_MedianFilteri ng
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(NV12,0x3231564e) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalSt retch
         Registry: OK
     DDraw Status: Enabled
       D3D Status: Enabled
       AGP Status: Enabled
DDraw Test Result: Not run
 D3D7 Test Result: Not run
 D3D8 Test Result: Not run
 D3D9 Test Result: Not run
-------------
Sound Devices
-------------
            Description: Realtek AC97 Audio
 Default Sound Playback: Yes
 Default Voice Playback: Yes
            Hardware ID: PCI\VEN_8086&DEV_24C5&SUBSYS_3189109F&REV_02
        Manufacturer ID: 65535
             Product ID: 65535
                   Type: WDM
            Driver Name: ALCXWDM.SYS
         Driver Version: 5.10.0000.5260 (English)
      Driver Attributes: Final Retail
            WHQL Logo'd: Yes
          Date and Size: 7/1/2003 02:05:36, 756444 bytes
            Other Files:
        Driver Provider: Realtek Semiconductor Corp.
         HW Accel Level: Basic
              Cap Flags: 0xF5F
    Min/Max Sample Rate: 100, 48000
Static/Strm HW Mix Bufs: 0, 0
 Static/Strm HW 3D Bufs: 0, 0
              HW Memory: 0
       Voice Management: No
 EAX(tm) 2.0 Listen/Src: No, No
   I3DL2(tm) Listen/Src: No, No
Sensaura(tm) ZoomFX(tm): No
               Registry: OK
      Sound Test Result: Not run