Back to the OP..
Going through all the effort of having everything backed up and reinstalling the OS might sound a bit intimidating.
If you want to see if a 'quick fix' will help.. and I don't know what you've done up to this point, I use 'Registry Mechanic' about once a month or so to keep the registry clean.
And, I found that defragging the HD makes a significantly better increase with XP than it ever did with Win 98.
Personally, I haven't reinstalled XP in over 6 years on my main machine.. ( 3x different motherboards and 3 different HDs, at least, since original install.. and I can't count the video and sound cards.. )
God only knows how much useless crud I have accumulated over the years.
Only reason I don't clean it up with a reinstall is that I have a nice, shiny new box of 'Full' Window 7 waiting to install ( which will wipe everything clean anyway ) once my employer authorizes our remote log-in software Win7 version.
Have fun.