I just recently recieved my USb from a friend who wanted a few files on it. I'd only jacked it up on my class's computer to transfer some files I needed. When he returned it to me, he said that it'd nearly killed his computer with over 2000 viruses. I have no idea where they could have come from since my computer is clean (scanned with AVG, Spybot and Malwarebytes Anti Malware all fully updated) and my teachers safely transfer files to the class computer without drama. Sure it has viruses, but nothing AVG can't handle.
My friend said that something uninstalled all his antiviruses and he had to redownload them as the pop adverts started coming in. The story sounded very Hollywood with a cliche ending but I'm too scared to check my USB myself.

What would your advice be? I'm considering just replacing it but my family's very pragmatic and don't like wasting these kinds of things.

Many thanks.

P.S. Yes, I know I shouldn't have lent it to anyone in the first place but it's always come home safe without drama.