Reformatting erases all the data on the hard drive.
Some people regularly perform a reformat and reinstall on their computer. This is akin to demolishing your house and then rebuilding it again on the same spot: all the cracks in the walls and creaky floors are taken care of and you get a nice new fresh coat of paint, but it's perhaps overkill.
A couple of less drastic things you can do to reclaim some of your lost speed is to defragment the hard drive
. Then do a full scan for malware using your favorite up-to-date malware scanner followed by a visit to Windows Update to install all the latest security and stability patches from Microsoft. Carrying our house metaphor further, this is like a maid, a building inspector, and a handyman respectively.