Did you download any files just before this started happening??
Have you run an AV scan? Update your AV & then scan your computer. Or try an online scan, there are links in the thread below.
WARNING: Viruses on network you should be aware of!

Or you can try scanning with AVG Free AV, it seems to find new viruses quite quickly. Make sure to update before you scan.
AVG free

If that doesn't help, try one of these sites, they offer specialised help for malware problems.
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