Win XP Failing to move past POST screen

[AnimalMother.]

Right, I'm visiting home and my parents have asked me to look into fixing up some old computers they have.

The main one is one of my old gaming rigs from ~2006-2009, running Windows XP home edition, ASUS M2N SLIDeluxe Motherboard, 2 x 2GB DDR2 RAM, AMD Athlon 64 X2 CPU, 8800GTX GPU. (Might be a little out on the specs but this is what I gathered)

So what happens?

Plugged in power, PS2 keyboard, USB mouse, DVI monitor (later used DVI to VGA cable adapter). Switch on the PC and several things can happen, at random it seems.

1. Fans and drives start up, internal case light comes on, nothing displayed on the monitor and no beeps of any kind.

2. Fans and drives start up, internal case light comes on, display has solid light as if there is something to show but is black screen, single beep.

3. Fans and drives start up, internal case light comes on, single beep and display is the M2N screen with "press del to run boot menu"

Here's the fun part, when option 3 occurs, I can't do anything. The keyboard doesn't function (I've tried all available keyboards in the house, mixture of USB and PS2). So I'm left sat there looking at the screen, at times I've left it for 1 hour to see if it'll move itself along but it never does forcing me to go for the power button (sometimes holding for 10secs to turn off, or sometimes just tapping it causes it to turn off).

I've been frantically googling stuff but haven't found anything of note. I've removed the CMOS battery, even replaced it with a new one, and that hasn't improved anything. Tried inserting the old XP disk in the hope that it'd try to boot from that and again only get the 3 different outcomes.

This is a little pet project to help my parents out but also has the added benefit of me having a rig that'll run PR when I'm home.

Any help greatly appreciated, also if you think of any questions for more information please ask and I'll do my best to answer them!

Regards,
Animal