1. This may be a problem with the RAM modules or other add-on cards. In case of RAM module problems, you will usually be alert- ed by beeps from the BIOS. Strip the PC down to the ‘barebones’, that is, just the power supply, motherboard (with CPU & heat sink), minimum RAM and display card. Switch the RAM module to another slot. Verify that the RAM modules are as per the specs defined by motherboard manufacturer. Use matched RAM modules from the same manufacturer. Try a different RAM. If everything is working you should boot up and see the message ‘No boot device’ or something similar. Power off, connect the hard disk and reboot. If the problem still occurs, this points to something wrong with the hard disk or the basic boot up of the operating system. Check the storage section and the Windows troubleshooting section of this guide. If the OS boots normally, then power off and start adding each add-on card one by one, powering up after adding each card until you isolate the culprit. Check the relevant sections for troubleshooting hints for those components.
2. It may be a BIOS problem. Check the BIOS section in this guide.