Dell E520 fails to boot

We have a Dell E520 (1 GB RAM) with Windows XP, Windows 7 and Ubuntu installed in a multiboot system.

The machine is 25 months old and has never given any problems. That is, until today when, on switching on,a succession of 4-5 beep sounds emerged from the tower and then stopped. The sounds were a cross between a beep and a buzz and lasted about 2-3 seconds each.

The monitor did not turn on and no Windows 7 login music was heard (Windows 7 is the default operating system).

I also tried booting to a Linux Live CD but again only the beep/buzz was heard and then nothing.

After leaving the machine off for about 2 hours, I tried to boot it again and this time it did actually get to launch Windows 7 and everything seemed to be going fine. Then I tried a browser search and the monitor immediately switched itself off and the beep/buzz sounds were heard. Subsequent boot attempts got no further than these annoying sounds.

I have to say I'm prepared for the worst (mobo failure or on the brink of failure) but would welcome opinions from more experienced users.