This is an FYI for you guys. What is happening is that these old computers were shipped with a minimal amount of MEMORY way back and now, with all the updates and crap that M$ does they have made them eat up memory like a big dog, hence the slow down. You can reload it but I'll bet you a "C" note that once SP3 is installed it will be slow as a snail again!! Think about it. IMHO M$ is doing this on purpose to try to get rid of XP and move ppl into Win 7!!
Now, what I've found is that most of these older computers only came with 128 or 256 MB of memory. With all the crap that's put in them now through updates I set the new minimum @ 512MB and recommend 1GB for optimum performance.
So check to see how much memory it has and if it's 256 or less go on fleabay and try to find a memory upgrade for that box of at least 512 and preferably 1GB. That stuff should be available for around $25 or so.