Hi I have heard of Malware causing this can you run a Virus scan and a check for spyware(use Spybot). There is a workaround use
alt-ctrl-del sequence. If the Windows Task Manager dialog box appears, and it usually does, select the File menu and then New Task (Run). When the Create New Task dialog box appears, type in Explorer and press the OK button. Windows Explorer should restart complete with the Start button.