Help! Boot Problem

First, some background/specs...
My P3 / 450 MHz has been running pretty much flawlessly for the past 4
years. However, shortly after installing a new Firewire card, I started
having increasingly severe problems. First indication was a rapid,
continuous beeping on booting. After resetting, the computer booted
fine. This kept happening intermittently for a day or two, but now when
I try to boot, I get no beeping at all, no screen, no hard drive,
nothing. The power supply and fan are running, LEDs light up, etc but
nothing else goes.
The computer has the following specs:
P3/450 MHz cpu
64 Mb PC100 SDRAM
Asus P2-99 motherboard
Award BIOS

What I've already tried:
I've tried replacing the memory (since I knew I had to upgrade anyway)
but no change. When I disconnect everything from the motherboard
(except for CPU), I get a repeating slow beep on booting but when I
added the memory, no beep, nothing. I've also tried with the CPU,
memory, video card and floppy drive connected (in case it was just an
output problem with the video card) but the computer never even tries to
access the floppy to boot.

As far as I can tell, the computer is hanging somewhere in the POST
process, but apart from that, I'm at a loss. Anyone have any ideas what
the problem could be or anything else that I could try to fix this problem?

Thanks for any suggestions,
Brian