Windows XP adds a lot of new features in the user interface that look pretty, but slow the system down. If you can live without the extras, you can speed your system up a bit. If your system is slower than 500 Mhz or has less than 256 megs of RAM, you may need help.
This will turn off all the "eye candy" that looks good but slows the system down:
Lo and behold you will have a system that looks a lot like Windows 98, but it will be faster than it was before. If you like you can turn things back on 1 by 1 in this same dialog until you get the look/feel/performance balance you like. Some options I'd suggest to turn back on are "show window contents while dragging" and "Use common tasks in folders." Have fun and good luck.
If you really want a faster system and are a bit more of a geek, you can always try Linux with a lightweight window manager like XFCE. Linux runs just fine on older hardware.