Hello and welcome to Bleeping Computer.
The problem computer may not be a total loss and may work once a clean operating system has been re-installed on a currently installed hard drive, so do not trash that computer yet.
There are two ways to get files off of a hard drive and a virus scan can also be performed on that drive in the process.
You can hook the infected hard drive up to another computer by installing into that computer as a SLAVE drive.
This would be the case of the hard drive was physically hooked up to the motherboard.
Another method is getting an external drive bay that attaches to the working computer through the USB port.
Inside the external drive bay is the connections for the hard drive.
Now it depends on the type of hard drive you have, it matters if it is an IDE/PATA type or a SATA type, because SATA and PATA have entirely different connectors.
I am providing you with a link to http://www.newegg.com
for some external enclosures
Now like I stated above, you can add your infected hard drive to another computer internally as a SLAVE drive and boot off of the operating system on the MASTER drive and either scan the infected drive, or transfer files to either your MASTER hard drive or a USB thumb drive.