I use Kaspersky products, so I'm not leery of doing this.
If you have a virus, you can boot your PC from a USB drive or a CD, scan and clean your hard drive (without booting from the hard drive) and the virus cannot prevent it's removal, since you are not booting from the hard drive and enabling the virus.
Here's a link to a free tool from Kaspersky. http://support.kaspersky.com/4162