Try removing any unnecessary programs which are running at startup:
Click Start > Run.
Type 'msconfig'.
Click 'OK'.
Click on the 'Startup' tab.
Click to remove the ticks alongside any programs you don't really need at startup.
Click 'Apply' and 'OK'.
Restart your PC.
If following Buenchico's advice doesn't help much, there are free guides to improving performance on XP and Vista at http://www.tweakguides.com/TGTC.html
Be warned they are rather long and can be a bit wordy because they are written everyone from novices on up, which means there are a lot of explanations in there. Ignore any sections which deal with cleaning up or editing the registry, not because the advice is bad, but because messing with the registry really isn't a good idea.
if it's become slow rather than always been slow for no apparent reason - don't start tweaking - something is slowing it down ... so do it by the numbers - you've covered the obvious .. buenchico's is next
do you have any networked drives - or shared folders?
what scanners(s) do you use?