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