Front USB jacks caouse re-start

Whenever I plug a USB device (or cable) into either of the 2 front USB ports on my computer, the system shuts down and restarts. They used to work fine, but suddenly there's a probloem. I've checked the connections inside and everything seems fine, but I don't really know what I'm doing. If I start the computer with a device plugged in, it reads fine. The only problem is when the computer is already running. The 2 USB inputs in the rear work fine. Any ideas? Thanks in advance.