The cheapest option would be to reinstall 7.
Download the ISO for your version of 7 then burn it to a DVD or use the USB/DVD download tool to create a bootable flash drive.
Boot from the DVD or USB stick then do a clean install of the OS. Warning, that will wipe everything out. If there are any files you want to keep then copy them to another hard drive or something.
After reinstalling Windows 7, run Windows Update. Then install all your programs again and copy over any files you might have backed up.