Check the following things (and I'd go in this order):
How much space left on your C: drive?
If under 2Gb, you may need to make space, XP needs much swap file space. If you have a second partition with more space, consider telling windows to put the swap file on that drive (especially if, as in my case, its also a faster drive), otherwise try making more room - remember that if you've deleted a lot of data but not emptied the recycle bin, the spce is NOT returned to the system, try emptying and see if your free space improves.
Is your dosk fragmented?
If you use a P2P file share program, the answer will be "yes", in anycase, the way to know is to run defrag. Disable screensavers and anything else that may periodically read the disk during defrag, then set it going and go out for a while
Is you system working around damaged disk sectors?
This is similar to defrag, but running the error checker. Be even more careful about disabling background tasks - leaving the screensaver on means that error checking never completes; I and many others found this out the extremely frustrating way.
Are you running a LOT of startup tasks?
Just pop up the task manager and look at processes, if its more than a page and a half without actual apps running, its time to slim if you can. While you're doing this bit, run a full scan of your virus checker, then AdAware, then SwatIt (virus, adware and trojan checkers respectively), and google for "xyz process win XP" where "xyz" is any process you are suspicious of in that task manager list - most of them will be windows, but one may be something dodgy. Dodgy processes eat resources, I have found.
Do you have enough RAM?
Now, if you've noticed that the machine is just plain getting slower over time, this is unlikely (unless its a case of "ME was fine but now on XP its getting slower", but I guess you'd have mentioned this...) If you have small amounts of ram (under 256Mb really) then try more RAM.
Is your CPU over heating?
See RAM above, but check vents aren't clogged with dust and fluff. You would expect your PC to hang if this was a problem, not get slow, however.
Do you need to reinstall windows?
This is a bit of a "well, I have no idea what's wrong" answer. But sometimes you can end up with a gently fragged install, which will work, but which is working around damage. Unfortunately in this case "damage" can mean orphan DLLs, or orphan registry entries. Each one of those takes time to process whenever its run across. You can run programs which claim to cure these problems - I personally don't trust them, but the alternative is format and reinstall.
Anyway, its likely that one or more of the top 3 solutions there will cure things. (in my case, I had a deadly slow down and moving the swap file plus a defrag made everything hunky-dory)
Good Luck!
