Check the event log to see if there are any pointers there to where the problem might lie. Boot into safe mode to see if the problem is there also.
However, I think that it is a trojan.
I recommend that you seek help here:
Geeks to Go! Tech experts answer your questions - there are some very, very helpful people who truly understand AV / AMW stuff.
Whether a virus / malware or other fault, backup your data (not any executables, screensavers, etc.) and be prepared to re-install Windows.
Assume that your PC is compromised, do not log in to websites where authentication is required, and change your important passwords locally and on websites, from an un-infected PC.
SD