Why cant my pc play these games

Alright heres the deal guys, my pc is old, like 2004 old, but the games i want to play are about that old too. My pc is bassicaly an emachine t3985 with a bunch of upgraded parts. This is the original specs of the pc, intel celeron d 340 (2.93 GHZ) cpu, 512 mb ddr 2 ram, 80 GB hard drive, and windows xp along with onboard audio and video. Soon after buying 4 years ago though, i changed the hard drive to a 120 GB hard drive, and added another 512 mb of ddr 2 ram. I also added a geforce fx5200 and i was able to play games such as morrowind, Half life, half life 2 and counterstrike. But now shortly after reinstalling windows, none of these games are playable do to imense lag. I have also updated all drivers. So i would like to really apreciate help from you pc gurus8).

yea, i turned all the setting to low in morrowind its still the same amount of lag, and i also changed my power supply to a 500 watt psu, i forgot to mention that.

With an fx5200 you will only be able to run them (if at all) on the lowest possible settings at the lowest possible resolution. So if you're not as low as you can go, you're going to have stuttering.

Really though, instead of spending time and effort trying to fix what you've got, you could be selling yourself down at the bus station in order to finance a new computer. ;)

The thing is, all these games were playable before i reinstalled windows.