It sound like you have a malware virus, to get rid of is easy,
First: when you start you pc press F8 constantly, until it bips, when you get to a manu select safe mode, it should be the fist one. If you have windows xp you should get a new user account, named Admin. select Admin and if you have vista or 7 you will not get the admin account so select you account.
Second: For xp: go to your start menu and click run, it should be on the right. Inthe box that opens type mrt this is a malware removal tool that come with your windows, when it pops up simply fallow the intruction.
for Vista or 7: type mrt is the serch box on you start menu.
that is it.