back up important files/folders to cd/dvd/flash drive.
use minitool partition wizard bootable cd to wipe out your data (full format of your hard disk drive,
all data are deleted, the hard disk is completely formatted). create hard disk partitions.
install fresh windows 7 OS, reboot if prompted. install all windows 7 updates, reboot if prompted
install antivirus, reboot if prompted. install AV definition updates.
do AV scan of your back up files/folders before copying them to the local drive.
minitool partition wizard is a free tool. if you opt for the paid version, it's your choice.
free minitool partition wizard bootable cd version instructions read
here.
if the website is not updated to the correct direct link, download bootable cd version
here, and burn the *.iso file to cd using the free tool image burn or nero or any cd/dvd burning software you prefer.