Booting problem

[Babu Kartik]

Dear Sir,
Please be kind enough to provide solution to the following problem.

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 all the
time or it should not display at all. There is no specific option in the
BIOS for booting from USB. If that means system does not support USB
booting, then it should not have come in the boot menu at all. Fact that it
appeared in the boot menu, means system   supports booting from USB.

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 from all the drives. Can there be a
problem with mother board?

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?

7.       Why doesn’t any error message displayed during boot process?