Did the hard drive show up in the Bios?
My uncle dropped his pc of last night with an "operating system not found" error. I went into BIOS and it kept rebooting before I could browse around much. I replaced his power supply about 4 months ago - so I figured maybe I'll try another supply and see what happens. I booted the pc with a known good (and powerful) supply and the same message came up (no os). I rebooted again but entered the Bios setup this time. I checked the boot order and moved the hard drive to the top ("removable drives" was at top). After rebooting, the PC started windows xp with no problem. I reconnected the old supply and the pc again booted fine. I replaced the bios battery and gave it back to him and haven't heard any complaints yet. Sometimes if the Bios battery is dying strange things can happen. I didn't test the battery. Just some thoughts for ya.