Computer problems

And of course, now that I reboot my server (I had turned it off to disconnect the monitor cable to install the KVM switch) I get problems. The server boots normally, but just as it gets to the login screen it reboots and the Bios then displays "Disk boot failure, Insert System Disk and press enter". The server had been going for several weeks between reboots.

Ideas anyone? I seem to recall the last time this happened it was a memory problem, which could be the case. That CPU/mobo/ram is pretty ancient. The hard drive is only 10 months old though.