Deskpro EN: not booting, beeping, caps lock blinking

Hi,

I've been using a fairly old Compaq Deskpro EN (the convertible
tower/desktop model, with a Celeron 400 CPU) as a file server on my
home network. Chose it because its fan is nice and quiet and the
machine *seemed* fairly indestructible.

However, last sunday, after running it non-stop for four months, I
needed to install some Windows updates and of course, it had to
reboot. I was fully expecting to see an Option ROM Checksum error - it
started showing that after the 3Com NIC was built in but it didn't
seem to hinder operation. Like I said, it had been running for four
months without any problems.

But now it didn't even get that far. Upon reboot, all I got was 3 beep
signals:
long-short, long-short, long-short-short. And then it just sits there
with the caps lock LED blinking.

Hunting the net I found that I could have:
- an option ROM failure
- a system ROM failure
- a system board failure (yikes!)
- a faulty gfx card

Now before I tear the machine apart (I can hardly reach it so that's
quite an operation) are there any of you out there that can give me a
clue as to what is the most likely thing to have happened? Anyone
recognise this behaviour?

Grtz,
Friso