If your poor computer is wheezing and gasping from malware ... that website you knew you shouldn't have gone to, or that download you should have thought better of ... I can usually clean up that kind of stuff without having to erase your hard drive. Sometimes not. Sometimes maybe you just want to start with a clean slate and reinstall the operating system. Sometimes you don't have a choice because you picked up malware that replaced some of your system files.
If your hard drive needs to be reformatted and Windows reinstalled, all of your personal files would be backed up to a cloud account (lots of choices for free cloud storage), and your partitions would be deleted. I can also work with your original disks is you have them, but I don't need them. I will also install Microsoft Security Essentials and make sure it's up to date.
Most computers with all their parts present and functioning, if there is a Windows sticker on the bottom, can be restored to near their original state.