Google and Microsoft are temporarily not quite on speaking terms with one another. The last update to Googles Avast anti-virus loads normally on Win XP machines, then when it reboots the machine it gets as far as the opening screen then reboots again. And again. And again.
The fix is to hit F8 early in the reboot process, start the machine in safe mode, delete Avast from your program set, then restart your machine.
I've been using Zone Alarm for a firewall and Avast for anti-virus, so when I did this, ZA took over the antivirus function. If you're not using two systems, the process may have you running naked until you can find a replacement for Avast.
I presume that at some point this bug will be addressed. I like Avast and have never had a problem up to this point. If you can load the previous version of Avast, then fine, just ignore the pitches regarding available updates until you hear that the bug has been addressed.