If you have an IT staff, call them. I see this problem all of the time. Depending on circumstances, a combination of Ad-Aware and Spybot Search & Destroy will most likey kill it, however, I recently ran into very similar problem that required more detailed work.
I DO NOT recommend you touch your registry unless you are well versed in computers. I cannot say this enough. You can really mess up Windows by deleting registry keys that the system needs. Also, please don't install anything or reimage the system without first asking about your companys IT policies. With my employer if you did anything other than call me (head help desk guy) you would be in trouble from the get-go.
If you don't have an IT staff, PM me and I'll help you. If you do, they should handle the problem easily enough.
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edited for sterness )