The reason the cast uses XP is probably the same reason I do. It's far more slimline than Vista. I know that Darren uses an Acer aspire one which has a single core and a max of 1.5GB RAM, for these reasons XP is an excellent choice. It's lightweight, easy to use, reliable and highly supported, compared to Vista which is slightly more "end user friendly", "looks nicer" but requires a superior set of hardware to achieve the same performance as XP.
That said, possibly the reason that I, darren, matt, the majority of the IT community running Windows and 95% of Business is still using XP is that XP does and probably will always have the best Performance to Hardware ratio for 90% of all users.
If you want to dual-boot linux, that's fine. With the GUI built into most modern Nix distributions it's ridiculously easy anyway, even setting up a bootloader is automatic. That's your call.
As mentioned above by other users I'd also recommend against getting a pre-made PC as they tend to be "non-downgradable" or at least "hard to downgrade" since 90% of all PCs now-a-days are shipped with SATA HDD's and XP will only support modern SATA drives with manufactuer drivers, which means a good traul for manufactueres drivers, or as I discovered buying a new hard disk (some manufactueres wont release their disk drivers for XP installs).
Not only that but in terms of Cost to Hardware you're going to get 50% more hardware if you build it yourself.
Just not true. Most end users can run Vista on 1GB RAM with absolutely no problems at all(Infact all of my family does). You should remember that most people just use a computer for Web/Email/Music etc.
If £50 is chump change then yes, however like I; I'm sure that a lot of the people reading the forums don't see it the same.