First of alll, sorry me for my bad english, traherom. I´m using XP home SP2 with the security center enabled, and I started to used Avast one month ago with the windows anti apy you describe and I have no incompatiblities, no crashes and no error messagas any kind. I´m not and expert but if you permise me I recommand you to use some system cleaner ( TuneUp utilities, for exemple ) to do a strong clean up on the registry, maybe is the only real trouble you have. I have not a long experience with avast but I´m very satisfied and agree to that effective and FREE ;D antivirus. I hope I´be been help you with my point of view. Greetings, fotudet