it's easier to use. it crashes less. porgrams are more supported (eg the latest 2 MS office wont work on win98 (3 if you include office2007 but not technically out yet). it will be the same as anyone else's PC they ask for help. drivers widely available and often automatic via windows update. system restore if they mess anything up.
usb flash drives, cameras, mobile phones via usb, external hard drives. any of those will all be recognised automatifcally. digital cameras (err and the internet) are responsible for massive uptake of PCs for previous non-PC users.
so all in all it will be a nicer and less frustrating experience for them.
and they wont have to relearn when they eventually upgrade to XP.
There is a conplete XP home PC for £213 here :
http://www.dabs.com/ProductView.aspx?Quicklinx=49N8&CategorySelectedId=11101&PageMode=1&NavigationKey=11101&InMerch=1I'm pretty sure you could get set up for XP with under £100 if you went 2nd hand even if you just strolled into a shop too (not PC world! a private / back alley PC repair shop - although they might not actually let you have a licence for XP for that price, i'm sure you could find a copy). Next port of call is ebay and you might be looking at under £50 if your selective.
Win XP minimum specification says 300MHz, 128MB RAM, now it wud be a bit slow with that but you can see, not many PCs wont actually run it, do you know the specs of you old PC ironside? I'd stick at least 256MB RAM in whatever PC and hopefully 600MHz CPU, that's still pretty modest but wud probably run fine for a beginners uses.