One other option, if considering purchase of a new PC, is to simply buy an external drive case (rather than external drive) and use the old drive in that. Once plugged in, you have your old "main" drive as a backup disk, and can pull files off it (so long as the drive is not physically damaged). You do lose out if software only worked with a particular old version of Windows, however, as most machines bought now will come with Windows 7.
There may be some (eg on Ebay etc) which are being sold with Windows XP Pro or with Vista, and while I'd rarely recommend it (from a legal standpoint) there are copies of Windows XP floating around as 'torrents' for free download, so a new (blank) hard drive for the existing PC could be set up fresh and the external (old) drive used to find old letters/ documents/ spreadsheets/ photos etc.
DISKS and these 'external enclosures':
In recent years, there has been a change from parallel to serial, so the cables are thinner (and looking at what's available at one online store, they seem mostly to expect serial, aka SATA drives).
One would need to check what cable is used to decide if the drive is SATA or not (the older drives were called Parallel ATA or IDE) - see en.wikipedia.org/wiki/Parallel_ATA and en.wikipedia.org/wiki/Sata
Next is the size of the drive - laptops use 2.5" diameter drives, desktops usually use 3.5" drives, so the enclosure must match...
Search Amazon.co.uk for "3.5 external hard drive enclosure" and you'll find SATA and IDE at perhaps 15 to 20 pounds, into which the old drive could be put, and viewed with a new PC once it is plugged into a USB port.
Lots of places to buy these enclosures - Amazon, Ebuyer, SVP.co.uk and so on.
Have to say (after my sister brought over her sluggish and old PC, plus an external case for a hard drive) that sometimes the design of external drive cases is odd or awkward - the ones I've used up to now had everything connected to a removable panel, but hers had an on/off button in the case with a (short) cable going to the removable panel - a major pain getting it to fit together!