Sounds like you may have downloaded some sort of Malware along the way...
Try running an AV and Spyware scan and see if they detect anything...

Here's some good freebies....

Anti-virus:

avast!

AVG free

Avira Antivir

Antispyware etc:

Ad-Aware __(FREE)

Spybot - Search & Destroy __(FREE)

AVG Antispyware (formerly Ewido)

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If you dont live for something...
You die for nothing...