The answer is not in your supplied clues. While many rely on your choice of AV and firewall I'd still give the EWIDO ONLINE SCANNER (see google.com) a pass on this machine and see what's up there.
Also, email me your HIJACKTHIS log since it usually reveals items and I won't have to interrogate you for many posts.
Bob
PS. Email is via my profile and HIJACKTHIS is found with google.com

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