Boot Disk Failure Insert System Disk

In the past 12 months I have bought 3 purpose built machines for my son to play World of Warcraft
All 3 have done the same thing, they work fine one day and give me the "Boot Disk Failure Insert System Disk"
After this message on the latest machine I re-booted and used Norton Ghost to install the latest backup, all was fine untill the machine was shut down for the night and next day got the same message at boot.
I entered the BIOS set up, the BIOS dosn't reconise any drives other than the floppy & CD so I gave up [ran out of cash for another machine] tried to start the machine a few times over the last 6 weeks, same thing, the today went to start machine to copy the message exactly & lo & behold it fired into windows.
Im'e fairly new to MGs but have been thru a malware clean on my machine with Chaslang over the last week [know how to post threads and upload stuff]
Would appreciate any help before I have to go thru another marathon and money session setting up and reinstalling on another machine

Regards and Merry Xmas to all from Craig [member handle 21st]