This one will depend on the type of issue your PC has.
If it was a nasty virus, all your files have the potential to contain a piece of the virus to reinfect the next PC.
If it is just a bad OS (which a virus could do, or a bad block on the hard drive or shutdown) then you would need to get that repaired first then the virus removed if one was present.
The USB drive was most likely a coincidence, unless you have an electrical short on your computer, which would be entirely aspect. This would also mean your motherboard has a chance at being damaged. Your data would be recoverable though.