This has not happened again but I had a small problem the other day when switching on my PC first thing in the morning. It wouldn't start up and told me that it couldn't find an NTDLR (or something like that). I restarted and hit delete to find that my boot order had changed from my normal SATA hard drive to an empty IDE second drive. I changed it back again and it has worked fine since.
What I cannot figure out is how the boot order could change. It was fine when I switched off the previous night. Does anyone have any idea what could cause this to happen. I don't appear to have any viruses as scans are showing everything OK.