Question Author
Thank you everyone for your comments so far. I bought the computer from PC Specialist and it came with Vista Home Premium ready installed. However, they also supplied the disc and the 25 character serial number. The only extra items of hardware I have added are the monitor (obviously!) and a printer, both of which are extremely well behaved. A couple of days ago I got the external hard drive already mentioned. This has built-in backup software which has copied everything apart from my program files (obviously, again!).
Other things like Internet (O2), Open Office, IrfanView, PaintShop Pro X2, I have in the past uninstalled and reinstalled and for these I either have discs for or I can download, so I'm not too concerned about these. What I am really after is "hints, tips and pitfalls for the unwary" so I don't get any unpleasant surprises either during or after the uninstall / reinstall.
Thanks again