Minesweeper or solitare is what you can run pretty much. The CPU is slow, and I bet you've got a socket 478 system, meaning that you can't upgrade to anything faster than a 3.4ghz prescott pentium 4. You have a chance of running some source stuff if you get a new graphics card. AGP cards (I'm assuming your system uses AGP graphics) are really cheap on ebay, a good AGP card (geforce 7800) which will complement your CPU pretty well can be found for £20 (you'll need a beefy(ish) PSU, I think 350watt with 18amps on the 12vrail was the minimum). If you can overclock the CPU, that will help a bunch. But you would need decent cooling, and decent powersupply and a decent motherboard to do this.