It is very easy, even if you don't do a lot with computers. Since you have AGP, you most likely don't have onboard graphics so that will make for no problems installing drivers. First go into control panel/hardware/devices/ and get rid of your old graphics drivers, then shut down. Well now you need to unplug your machine and bust open your case and the AGP slot will be brown and at the very top. Touch the power supple to discharge any static and unscrew the screw on the intel and gently pull out. Gently slide in the ATI and screw back on, replace the case and plug back in. Start your computer and plug n'pray should automatically recognize your card. Click cancel when plug n'pray tries to install microsoft drivers for the graphics and when you get to windows pop in the ATI cd and install ATI's drivers. You should be set. Let me know if there are any problems.
[Edit: (I always forget...) Welcome to TSG!)