I have an old, assembled desk top loaded with win7 ultimate. It has got 80 GB hard disk and 1 GB RAM. Earlier there was XP. That time CD drive was not working at all. Subsequently I upgraded to win7. After few days suddenly I discovered that CD drive was working. I could burn files also through this drive. Though booting was not possible from CD. After few days suddenly, system stopped loading win7. I wanted to re install win7, but was not possible without the option of booting from CD. Whenever I tried to boot, one welcome screen and another screen came after one beep each and then it halts with a blank screen. I prepared a bootable USB drive with XP with an intention to install XP and thereafter upgrade to win7 (instructions for XP alone was available in the internet). After preparing the USB, I checked it in another system to see if it was working. In the BIOS there is no option to boot from USB. So I disabled all three boot options (Floppy, CD, HDD) and enabled “other” boot option and tried to boot from USB. Out of my nearly ten attempts, only twice, USB drive was displayed in the boot menu as a boot option, though it did not boot from it. Presently system does not boot either from HDD or from CD or from USB.(Floppy drive is not active since the relative cable is missing). My question is:-
1. Why should the boot menu display the USB drive only twice? Only in case of a loose connection such things can happen. I believe there is no question of any loose connection here. So either it should display or should not display at all.
2. Why does not the system boot? I have a feeling that perhaps there is a component which is responsible for detecting drives or booting in general was failing slowly because of which CD drive was not working (when XP was there) and there after it was not booting from CD and again there after it stopped booting altogether.
3. Someone told me that, in case of RAM failure, system will not start the boot process at all. Here since boot process goes few steps producing two screens and two beeps, perhaps it is trying to boot but why unable to detect the drives.
4. There are two separate cables for hard disk and CD drive. CD drive is primary and hard disk secondary. When I exchange the cables, it is reflected in the BIOS that is CD drive becomes secondary and vice versa. That means BIOS is recognizing the drives.
5. I tried with different boot order in the BIOS as well as by making the BIOS setting as default.
6. I tried the installation CD, so there is no question of whether the CD is bootable or not. Similarly if MBR is corrupt, it should give problem only for HDD booting and not for others like CD and USB. Then where is the problem?

Kartikeswar Senapati, October 2013

Point 1, not sure. Looks like some intermitent problem.
Point 2, maybe there was some misscconfiguration or the CD was disabled...
Point 3:
A RAM problem can cause a boot failure at any point, it depends of exactly WHERE (at what address) the RAM error is situated. I've seen RAM failure cases when the boot process seems to proceede normaly, just to have the OS miss start and cause a restart loop. In other cases, it can cause random crashes and freeses, including some BSODs.
Total boot failure caused by faulty RAM only occurs if the fault is situated in the first few Kb of memory. In this case, you can't even access the BIOS configuration.

Your problem seems to be intermitent, making it realy hard to diagnose.
Try using some other RAM, or, if you have more than one stick/bank, removing some of it. If removing part of the RAM or replacing it solve the problem, you have a RAM problem and need to replace the defective RAM.

Try removing and replacing every easily removable component. DON'T touch the CPU, but remove and replace the RAM, every expantion cards, all power and data cables. Some component may be unseated or the contacts dirty or coroded. This process ensure that every thing is properly connected and any dirt/corosion rubbed away.

Try leaving the computer ON a few minutes, then perform a boot attempt.
If it works, it can indicate a cracked or broken weld of some component. In that case, the motherboard is no longer reliable. Finding the fault is near impossible as it may be located inside the casing of an IC circuit.

Point 4, you are right, your BIOS is working correctly and, at least, the low RAM is problem free.
Point 5, normal diagnosis procedure, good work on your part.
Point 6, if the MBR was corrupted, it would have been impossible to install Windows 7 without having the install process perform a full formating of the drive.

You are right that it won't affect booting from some alternate source.

Electro, October 2013