I wouldn't trust a computer that's been infected with "a number of viruses". I would definitely not want a disk burnt with it or a USB stick from it in my computer.

Zero format the HDD, period. Like Neil Young sings in a song,The damage is done - well, he doesn't sing about computer viruses, but it's comparable, Neil Young - Needle and the Damage Done (Live at Farm Aid 1995) - YouTube

You need to be sure it's clean, ok?