i reccomend getting a new computer. maybe like a dell inspirion
Pc issues...
Okay, apply new thermal paste too if overheating is the problem(if you did not do this 3-4 months ago ofcourse). As for the OS... install WinXP or even better, a linux distibution, it will run much better. (If Vista even runs on your machine, is a miracle)
If you do not have finances for a new computer, why not spend a couple of euros on upgrading it with cheap used parts?
Win XP are still supported for a short time as far as i know. Like I said, Linux should be an option for you with a PC like that. And upgrading does cost very little, got 2Gb of DDR2 RAM for 10 euros a month ago.
those specs.you should just toss the thing and get a new PC, if you build it your self you can get a decent rig and save a lot of money.

!I run windows vista