Beware if you are running the Online Armor firewall. The recent automatic avast program update made a mess of my system (XP Home SP3). After much work, I figured out that even if I shut down the firewall during avast install, the reboot process after initial install causes some kind of conflict (presumably because OA is trying to start up again). Only solution is to uninstall OA, or prevent it from starting on boot, until the avast install process is complete. I didn't try safe mode, but may be the best solution.
The message is; if you run Online Armor, do not have avast automatic program updates turned on.