Hi,
Scan is completely depending upon the file stored in the system.
Do we have any other security program in the computer (like: Spybot-S&D, Norton in the computer).
Did you receive any kind of alert/advertisements or pop-ups in the computer?
Steps can be tried:
Delete temp files from the windows as well as user temp. (Start-Run-type (temp) delete all the file and then Type (%temp %) delete all the file)
Try to run the free versions of tool:
http://www.malwarebytes.org/mbam.php clean anything it detected.
If the issue persist check with McAfee technical support chat department for further trouble shooting steps
http://service.mcafee.com/ServiceSel...S&pt=1&st=CHAT