Thank you for the replies
RE: s-n - Unfortunately, I think I'm able to talk the talk better than I am walking the walk as they say. I hadn't gotten as far as looking in to the bios, as it's been about 8 years since I built a computer for myself and have had to have a look in there. Much like getting behind the wheel of a car after having spent the last 8 years as a paraplegic - one is naturally hesitant to start the engine.
I have, upon reading your reply, been in to the bios. It was scary at first, but I was able to fight the urge to put a hammer through the monitor and yell "witchcraft" and look at the 'Drives' section. As I have 3 drives already (two disk drives), the old HDD was assigned SATA position 3. As suspected, the drive was turned off. Whoops. Only, upon turning the drive on, and trying to boot - my computer would try to boot from this drive, which is not what I want. Partially because I've been having problems booting from it anyway (Windows refused to load from either 500Gb mirrored drive on my old computer, so I suspect there's a problem with the Windows install) and partially because it's meant to be a slave to my 1Tb HDD with the 64-bit Vista installation.
Even when offered the choice of boot sequence, and manually selecting the 1Tb drive, it still tried to boot from the 500Gb. Personally, I think that's a bit weird. In order to get back in to Windows, I had to turn the 500Gb drive off again.
RE: kword88 - I have indeed looked for the drive in disk management, since your suggestion though. It wasn't there *sadface*, even after a disk rescan. As for a "raid controller", you've lost me. The last computer I built was all IDE-based - that was computing for dummies and even I could get my head around the master/slave system there.
I'm still about as lost as a nun in a brothel, but you've both been a big help thus far and hopefully I've elaborated enough to allow you a clearer idea of what's (not) going on with my computer atm.
Further assistance would be highly welcome!