PSU problem?

Hi,

Ok, my computer has become extremely temperamental so I'll just post
what's been happening and hopefully someone can shed some light and
advise me.

This all started when I came back to my pc and pressed escape for some
task I've forgotten. The computer was unresponsive and something in
the hardware just let out a beep when I pressed. I thought I'd just
leave it alone for a while, but within a few minutes my pc was letting
out a continuous beeping - I left it a few more minutes and then just
restarted it by the button on the box

When I tried to turn it on again it just froze on a blank screen. After
repeating this seemingly endlessly, I noticed that it froze on the
blank screen immediately after the bit of text which said "verifying
DMI pool data...", and also that as the computer started there were
beeps (I thought it was three short beeps, all very close together).

Not knowing much about hardware, I cracked out the Phillips screwdriver
and opened the box up. I poked around a bit, didn't really change
much, and then wired back the monitor and the mouse and started it up
again. This time it worked!
Not wanting to tempt fate, I immediately made for my important files
and tried to get them backed up onto a DVD. For some reason this
wouldn't work (I think the error message was something very generic,
like "error, XP cannot copy this file).

I turned my pc off by the button on the box, and tried to turn it on
again, but this time it was totally unresponsive. Nothing happened.

After a minor heart attack I unplugged all the external stuff again,
and then rebooted it. Again it worked, and instead of backing up stuff
through DVD, I plugged the modem in and backed it up via the network to
my Dad's shiny new laptop. Half way through messing around and
looking on the net for a solution, my keyboard (which was plugged into
a USB port) locked out.

I rebooted (think I just restarted through XP rather than going through
the whole rigmarole of unplugging everything). This time my keyboard
worked but my graphics card didn't. I was seeing XP in some
ridiculously small resolution with about 16 colours. It occurs to me
now, that possibly in my earlier attempt to backup, there had been
other components that XP had been trying to install such as my TV card.

Next time I restarted, I didn't risk using my keyboard - I just had
my monitor and my mouse plugged in at the back, and it worked (sort of)
ok. I haven't tried since (been busy with other stuff, and my PC has
been off for a couple of days).

Does anyone have any idea what I should do?

N.b. I *think* it might be a PSU problem. This would seem to fit,
because it explains why my DVD writer drive wasn't working, why
sometimes components aren't working properly, why my PC sometimes
won't turn on. I also have had a kind of continuing issue with my USB
pen drive not connecting properly (first XP recognises the new
hardware, then I click on My Computer and see it, but windows kind of
freezes, and is pretty unresponsive until I unplug it). I've had a
similar issue with my digital camera, which is also connected via USB.
It's also been suggested to me that it's a problem with my
motherboard.

Obviously if it's a PSU problem, I should be able to just order a new
one, but I want to be sure that it IS that problem, before a I buy a
new one. Thanks