Most new hard drives (boxed) come with the metal bits and screws to attach the drive into a 5.25 inch bay. As you are going to use a HDD from an old computer you will need to purchase the parts. Most computer stores stock them including MAPLIN and MICRODIRECT etc.
Thank you Seedie and Seashanty, I'll try those sites.
One other thing has occurred to me. The old disk has Win98SE on it, could I leave that on and by changing the start-up sequence in the BIOS have a sort of dual boot for use in emergencies if the main drive misbehaves?
The answer to your supplementary question is yes, you will be able to have a dual boot system in principle, however, it depends on what your main OS is (ME, XP, etc.) as to whether you will need a third party bootloader to manage the booting of two OS's. XP and W2K have their own bootloader, but 98/ME does not. You should not have to alter the BIOS at all. Post back if you need more info.
I have ME and 98 (old disk). I have already given my daughter the DVD drive and have tried out the system (only for a few minutes) with the old drive precariously balanced in the bay and it works booting from IDE 1 (slave).
I don't particularly want a dual-boot I just want to make a copy of year's-worth of data, e.g. music, video and pictures. In the past I've used CD-Rs which work well but I also had the old disk from a failing computer and decided to use it.
I'll take up your offer of help to set up a dual boot providing it doesn't entail reinstalling windows - too much pain to get all my options set up.
My thanks to Big Elf for his kind gesture to send me (free) an old caddy.
I have used this Forum for what seems like years and always thought it was great, now I know that it is also a kind place to visit.