SBS **WON'T TAKE WINDOWS UPDATES"

I am inheriting a SBS that will not take SP2. It tries to install it and then when it wants to write to the registry, it craps out! I've tried looking at logs, etc and I'm having a hard time finding the problem. I believe it's a registry problem. The AV is outdated also. Do you think it could be something to do with a virus? I need to purchase the AV software, but is there a quick download for AV I could use?

thanks,