Sounds like you have a virus, read this thread.
WARNING: Viruses on network you should be aware of!

Update your AV & scan your computer! If you don't have an AV program, these are free (update before you scan).
avast!

AVG free

Avira Antivir
Or, there are links to online scans in the 'viruses on the network' thread.

Always make sure that you're using a good AV program. Keep it updated & scan all files that you download, before you open them

If you're still having trouble, try one of these sites for specialised help.
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