Desktop Vista Graphics Help!!?

I have a Dell Inspiron desktop (not sure if you need the model to answer this) with Windows Vista. When I got the computer, I was able to download many games such as Sims 2 (the entire series), Roller Coaster Tycoon, Zoo Tycoon...Basically, a whole lot of games without it affecting my computer speed or performance. One day my computer crashed and the only way to fix it was to reinstall Vista, which I did. When trying to re-install my games, I got an error message saying the computer couldn't support them. So I found the ATI Radeon disk that came with my computer (supposedly the disk that was used to install it initially). When I inserted that disk, another error message came up saying that it was not compatible with Windows Vista. Anybody know of this kind of thing happening? If I install Windows 7 alone, does it have a built-in graphics system? Should I buy a graphics card? Any suggestions?