Welcome to the forum luiz.

There is a bagle rootkit making the rounds that could be the cause of your problem (possibly two variants, actually).

Please try scanning with F-Secure Blacklight

http://www.f-secure.com/blacklight/

and if that doesn’t solve the problem try a Panda on-line scan

http://www.pandasoftware.com/products/ActiveScan.htm

Please let us know if this works and if you are able to isolate a sample of the malware.