help me

i am running win xp sp1 i think it has a virus because when i start it i get to log on put in pass word it then says loading personel setting then straight away says logging off and takes me back to log in can anyone help ???

First off start in safe mode then get Avast to schedule a boottime scan. Then run all your anti spy apps in safe mode. Then get SP2 and all current updates. Welcome to Avast

how do i start in safe mode?

SafeMode (repeatedly press F8 while booting):
http://support.microsoft.com/default.aspx?scid=kb;en-us;315222