Yes, replace MucAfee immediately with MSE. It is the worst AV of all and causes endless problems.
Then work through these steps to Clean Up Factory Bloatware
and Optimize Windows 7
which may solve your performance issues.
If you decide to Clean Reinstall follow the steps in the link Golden provided above which explain how to decide which partitions to keep, if any. You can create and format your desired partitioning during install if not.
If you decide to keep this install then post back a screenshot of your maximized Disk management drive map using the Snipping Tool in Start Menu. We can advise you better then about adding a partition.