If there is vital data on the old PC, take out the hard drive, drop it in a USB external hard drive case. Plug the drive into either a Mac or Linux machine or a Windows box with current up to date virus software (I suggest Malwarebytes) and scan it from there.

Alternately, find a bootable CD with a mal ware scanner built in, (I think something like this may be available from the malwarebytes site) boot from the CD (or USB stick) and let the software do it's magic.

I hope it's sorted easily.