Unfortuantely, it seems that your computer has been infected by a virus. Have you been using a firewall and antivirus? If you are going to use the internet, (especially with internet explorer which is extremely insecure) a firewall and antivirus which is kept up to date is a must.
Removing a virus once it has gotten to that stage is extremely difficult and time consuming, so I'd have to recommend that you reformat your computer and reinstall windows. While you are in safe mode, try to back up all the files you want to keep. Before you restore the files back, make sure you scan then with a antivirus first.
And next time you go onto the internet, use firefox. It's a lot safer. =)
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