Desktop

you probably need to flash your bios..

Hello,
Try running FDISK /MBR this will rewrite the master boot record. How do you get it to boot? Just turn it off and then on until it boots? You may want to do a DOS level scan for viruses as well. Hope this helps!
Chuck

At this stage of troublshooting and you cannot clear up the problem,use a boot disk to see if the partitions exist on the drive
Insert boot disk at command prompt type Fdisk
Choose the selection to view drive info,
If the partitions are there then I would suggest a new hard drive

This is what you need to know..
When you power on the computer does the HD spin up and is ready?
I have seen this problem and it is random to the point that the HD does not spin up and come ready every time the PC was powered on.

When you haveyou CMOS set to auto detect HD and the HD does not spin up there is no HD in the SYSTEM configuration.. In CMOS set the PROPER drive using AUTO IDE DETECT..
Now when you turn on you system the system will check for the hard drive and wait for response.. If no response from the HD system error is posted on the screen and you will be able to remove the case cover with out powering off the PC and check to see if the HD is spinning or not.. Either way you probaly will want to remove the HD as it sound that NOT reliable for boot operations..

ALSO as #4 indicated a weak or bad power supply will create problems in the boot up process..
I put my $ on the HD..