Log Off Windows Problem

Hello, i have a problem with the Windows....ok i log on all is good, after 5min i get "Log off Windows" spam.... i v formated the Pc several times...the same problem,i have Avira, nu virus detected evrey thing is "ok", i v puted paswords on evry possible acount,iven on the Safe Mod Administrator....still have the "Log off Windows" spam
Any ideeas what i mast do?
Sry for the bad english