Similar to this related question, I came home and found that my media center pc showed this message,
Disk boot failure, insert system disk and press Enter

Before I do anything with the (defective?) harddisk, what are the best first steps I can take to fix this with a minimum of damage?

Normally the machine (Win Vista) is always on and never reboots by itself (Windows auto-updates are disabled too). Something must have caused it to reboot, though I'm sure we didn't have a power outage. The machine can't reboot on that disk, but it will boot on another disk I just plugged in for testing.

I haven't changed anything, or even touched the machine, for several days, and it has been running fine until now. I did replace the power supply some weeks ago, because the old one suddenly stopped working. It has been working fine with the replacement PSU.