Service Pack 2 won't install

running windows xp pro.....have the disc for service pk2....it begins the install checking the files, backing up and then begins the install process....it only runs a short bit and I receive a small alert that says
setup error
unable to read from or write to the database
later I see a window that warns me that service pk2 did not complete installation.

I have restored my computer to an earlier date....tried downloading the sp2 as well as running it from the cd....same thing. I have disabled and finally removed the virus scanner, the firewall is disabled. I am a loss as to what to try next. this sp2 cd installed on another computer running xp home just fine.

If I don't get sp2 on I can't use new avg 8.

any ideas what might work??
thanks
Katy