I had same problem in January, tried numerous uninstall / reinstall, all failed. At one point Avast had the Windows XP Registry inaccessible and I was getting numerous errors trying to open any software, run CMD, explorer, everything. I was finally able to (after several reboots) open a web browser, I used the link to the Avast web site uninstall it finally removed the product. Rebooted and all the registry related errors went away. Reinstalled Avast and was protected again. This lasted two weeks. I rebooted last night and after the reboot the Avast icon had the red X in it stating I was unprotected again. Another hour this morning of repairing (unsuccessful), uninstalling, reinstalling (unsuccessful), another un - re, rebooting, still red X but clicked Start services and after two minutes Avast woke up and now says protected.