|I know of no virus that causes your snapshot symptom or screen dimming.|
Off the top of my head I don't really suspect a virus. Those that survive a reformat (which are few) usually give much more severe symptoms - you really know you have a virus around. Sometimes malware scans can reveal relatively minor issues (like cookies) - not "everything" they report is particularly serious.
Whatever, you say you did a malware scan. Did you include the following programs?:
You could also run this root kit finder
Best not to Run the downloads but Save them to a folder or the desktop, then double click the file to install/run them.
The above will usually find a nasty, even if you need something more powerful to fix the difficult ones. If nothing serious is reported then I doubt you have a virus.
Always pop back and let us know the outcome - thanks