|1st, 190MB isn't enough RAM to run XP decently & it's no where near enough for gaming. You desperately need a RAM upgrade - minimum should be 512MB for normal use, 1GB+ for gaming.|
2nd, the 6600GT is a bit outdated but it was a good card in it's day. If it requires an aux power plug connected to the card, make sure you hook it up. And make sure to disable the onboard video in the BIOS & return the shared RAM back to the system. Set the "initial display" to the new video card. The monitor MUST be plugged into the video card. That may seem obvious, but some people forget to do it.
3rd, installing drivers is extremely easy. All you need to do is download the correct one, then double click on it to start the installation. Here's the lastest version that supports the 6600GT: