1- is this a pre-installed windows 7 or did you upgrade a PC?
2- What was the origional anti-virus on it before you tried McAfee?
3- Did you properly remove the previous AV using the right uninstaller?
I would suggest Avast to be 100X better that Microsoft Security Essentials....much better on-line protection with NO scheduled scans neceassary, etc...your choice.
Some HELP in posting on Computing.net plus free progs and instructions Cheers