A new hard drive is one option with how cheap hard drives are.
Another option if you want to save money, is to back up any important stuff you want to keep.
Figure out who makes your current hard drive, there are a few ways of doing this. I really like hirens boot cd, found here
http://www.hirensbootcd.org/. I'll run one of the system information utilities on the disc and it will list the hard drive manufacturer there. You can also go to control panel, system, device manager, disk drives, and it might say your hard drive manufacturer, or hard drive model number, it varies. That's one of the reasons why I usually just pop in hirens boot disc and use their system info utilities, much more detailed information can be pulled from your pc.
After that, I would zero fill the entire hard drive using the hard drive manufacturers utility. Zero filling the hard drive is like reformatting, except better and more thorough.
After that is done, you should be all set and able to reinstall windows using the windows 7 cd.
*One more important thing to note, in my opinion. On hirens boot cd, there's a program called license crawler. This searches your pc and creates a list of programs like microsoft office, and provides you with the cd key that is on file in case you don't have the cd key anymore. So before you wipe your computer, it's important to make sure you have cd keys for any software you have purchased.
*I should also mention that sometimes running programs like malwarebytes in windows safe mode can find malware the had not been previously been found when running windows in normal mode. I've had some hard drives that were so badly infected that I ended up pulling the drive, and scanning it as an external drive on a different pc using something like malwarebytes. Once the operating system has been booted up, it can be very very hard to get rid of some of the malware.