i read one similar that suggested going to run,cmd and doing a system file checker to make sure that ntdll.dll was not corrupted. in the comand prompt type in , sfc /scannow, note there is a space between the c and the /
also try this get hijackthis and scan and post a log here ,don't fix anything untill i have a look at the log ,we will be looking for a startup item that may be causing it .