Computer Lagging at boot screen

[Death Row]

My cousin recently gave me a PC of his, which I'm having some pretty annoying problems with.
It's a core 2 duo PC, yet the boot time - is ridiculously long.

It get's past the POST screen, and then get's stuck at the blue "HP invent" screen. It stays there for about 5 minutes, then it changes to a screen saying saying there are new hardware changes, so I press F1, and it seems to load, then goes back to the blue screen.

It does this for about 15 mins, then finally boots up....to vista.
The OS then also runs extremely slow as well. A reboot - and it's exactly the same again.

I've really got no idea where to go from here, I'm considering formatting it - but I don't see what benefit that could do considering the problems start before the OS kicks in. I have tried resetting the CMOS, and it did boot up faster that time, but one reboot later and it was back to normal.

I've got no idea what to do now, so any help would be greatly appreciated.

 

[Death Row]

So I went ahead and formatted it, and it's still the same. The weird thing is, on occasion it will boot up fine, but other times it won't.
I'm considering updating the bios, although I dunno if that will make a difference.

As alway, any help would be greatly appreciated.

 

[KSoD]

Replace the CMOS battery. sounds like it is dying.

 

[Death Row]

Well, I've updated the BIOS, to no effect.
I've looked up the battery I'll need, so I'll try that next. But, a little googling didn't really provide much support for the battery being the culprit...surely if the battery was dead, the bios would be recalculating the RAM every time I booted...