You have some malicious software in your machine - Malware - with evil intent.
I don't know windows 7 but you may wish to ask your machine to go back in time to a restore point - a software configuration some time before your problem arose.
Give serious consideration to deselecting everything in the startup list.
Try to find some pointer indicating the name of the malicious software and search for solutions on the web - take care - some alleged cleaners are malware themselves and will also ask for payment. Taskmanager may give you the name of the naughty software.
Consider using Firefox as your browser rather than Internet Explorer - avoid file sharing sites - and think about investing in some protection software.
I use Spybot Search & Destroy (free) to get rid of malware - some say it's not very good but it works for me. If I go to a site and it tries to change something in my machine Spybot lets me know and asks me if it's OK for the change to take place.
Good luck...
Peccavi
February 2010