But like how old? One thing about XP is that as it has improved over the years the minimum requirements have increased. So has the requirements for software that we're stuck using like media players, Adobe, Flash and browsers. Most browsers leak memory like crazy. I mean just to have a decent running computer you need 2GB of RAM and a processor that is less than 4 years old. If you've got that covered then your problems are 100% software / OS related. Most computers I run across are bloated with garbage and stupid chat software and widgets that tell you the weather on your desktop. They also have viruses, malware, crapware and spyware even though the scans come up clean.
If you've ever gotten a virus you need to reinstall the OS and start over. I have three computers here at home, two of them are pretty old and one is running an old single core Intel Atom at 1.6GHz and 1GB of RAM (I use it to stream movies to my TV), it has XP Home. The other XP Home box is going on four years old but was a higher end system. Then I have my laptop which is newer but a very low end Celeron system. I run AVG (and no other preventative or scanning software), VLC media player and FireFox plus standards like Adobe, Flash and Silverlight. I have no other crap on my computers. I leave them all on 24/7 and I never notice any slowness or anything, even on the Atom box. I know what I am doing and I'm the IT guy at work but I don't work magic. I'm just careful.