GAC is the global assembly cache, open it by hitting [Windows+R] and typing in c;\windows\assembly
You can drag files onto it to install files to it. but if it says access denied, you are out of luck. If you are writing a program, your setup application should install those assemblies into the gac.
Rick
Vista Home Premium, MCE 2005, 3 Sony VBG-XL1B 200 DVD Changers, Optoma Projector, Digital Cable (Time Warner), ATI All In Wonder 9600XT Tuner, Sound Blaster Live! MCE Remote, ATI Remote Wonder II, two XBOX1 Extenders, two XBOX 360s