Uh 2001? I'm assuming it still has AGP, DDR1 (or less), and a REALLY old CPU socket. Probably the best you could do for that thing is add more RAM.
However, I'm sure any $500 laptop could probably outperform that PC. Seriously. Just get a new PC. Even the cheapest thing money can buy will do, theres NOTHING you can do for something that old.
You can probably transfer critical files to whatever you get, its not that hard IMO.
Best of luck
i guess you could get an x800 or something for it.
and get more ram, idk where you'll find ram that old.
and get a faster p4.
but like doomturkey said it's probably not worth it. spend 500 dollars on a desktop and it'll be 10 times better. or buy a cheap 300 dollar system with a decent processor and put in a 100sih dollar graphics card or something.