In my opinion, since you say the Apple system is a desktop, it sounds like a faulty power supply. If that thing is new enough to use an ATX power supply, you're in luck that it will be easy enough to fix. But since the G3 straddled the change at Apple, if it's some older proprietary system, you're going to have to scrounge ebay and craigslist for a replacement.

Same problem for the Compaq, only since that's a laptop, it almost certainly means that the motherboard has to be replaced. If the thing is under warranty, get Compaq to send someone around to do it. Replacing laptop motherboards is fundamentally similar to desktops, but it's often a bit more involved, and is a game of "find the screw that's holding the bloody thing in place."