I originally posted this as a response in another thread. I hope you find it useful on its own.
Prepare all your CDs and utilities. Double-check that you have all serial nos etc. I tend to have everything in a directory c:\install. If you're running FAT32 and have a DOS7 bootdisk and the space available, copy the whole XP CD to its own subirectory (e.g. c:\install\xp); advanced users may like to take the opportunity to slipstream SP1, add extra drivers, etc. One oft-used system is to have all your install sets on C: as a FAT32 partition but have D: as your XP partition running NTFS.
Save all your settings as above.
Save all your application settings (this varies per application).
Boot the CD to a command prompt (the Recovery Console).
Rename c:\Documents and Settings
Rename c:\Windows\win.ini
Rename c:\Windows\system32\ntoskrnl.exe
Rename c:\Windows
We now have an OS that will not be picked up by the reinstall process but can be made active if needed.
If using a CD boot from the CD and install XP from scratch.
If using a DOS7 boot disk, boot from the boot disk and run c:\installdir\winnt /s:installdir. It helps to have HIMEM.SYS and SMARTDRV.EXE loaded but is not necessary (there are other command-line parameters which may be of interest). XP will now install.
Reinstall all your apps. Because most of the files are already in place, this should be much quicker than installing for the first time. Some apps (e.g. XNews) may not need reinstalling at all.
Set up your users and copy back the necessary parts of the old profiles - indeed you can simply copy back your old profiles if they are not corrupt (if this doesn't work, simply delete the copies).
Restore your application settings.
If it all goes horribly wrong, simply boot to the command line, rename the new installation directories and undo the renaming you did to start with and you are back to where you started with no harm done.
