Microsoft's slightly odd Malicious Software Removal (MRT) gets downloaded and run every month - they apparently produce a new one each month. Granted it does check for a handful of nasty viruses but there are hell of alot more of them out there... However "The Malicious Software Removal Tool will send basic information to Microsoft if the tool detects malicious software or finds an error". It can stopped from reporting home that "basic information", though it's a little hidden in a KB article. Basically run regedit and create a new key:
Then create a new DWORD item under that key named: DontReportInfectionInformation
with a value of "1"