Don’t know what might have caused this to start happening, but you should make sure you have updated antivirus and spyware protection. If so, do scans in safe mode.
Check your startup programs for anything non-standard. Use Startup Control Panel from www.mlin.net/StartupCPL.shtml.
Check [HKEY_CURRENT_USERControl PanelDesktop]
and change AutoEndTasks=1 to fix the symptom.