have you tried ctrl-alt+del? if not, do this and look in applications, might be in there and end that app, also, try looking in processes but be car5eful, look for anything running that shouldnt be and end process. you could also try this:
start/run then type: msconfig , then go to start up and see if anthing strange is lurking there
if u have tried all ofo this, i apologise in advance, your question was a bit short on what u ahd already tried!