Can a magnet destroy the hard drive inside a PC? If so, how strong a magnet does it have to be? I ask because I dropped a refrigerator magnet onto my PC and I'm worried that it might be damaged.

Yes, a magnet can destroy the hard drive inside a PC, but you need a much stronger magnet than the type found attached to a refrigerator magnet. Nib Magnets

Here's a video of a Toshiba notebook being destroyed by a magnet. The magnet that caused the destruction is off a fishtank cleaner.

Want a magnet that is guaranteed to destroy a hard drive -- or for that matter, a CD/DVD drive? You need something like this 2-inch N52 neodymium disc magnet that has a pull force of approximately 450 lbs. Get your hand clamped between this and a piece of magnetic metal, and you could lose your hand. This won't just erase the hard drive, it will wreak it, along with the CD/DVD drive and any fans.

Neodymium Magnets Ring I know, I've used one!