To answer original Qs:
Often not cost effective to build PCs now - you simply can't get components as cheap as HP/Dell/Acer. If you have specific requirements, thats different.
You can set up a server. MS Servers are a bit dear for home use unless special requirements again. MS Home Server, afaik, is only available preinstalled. Servers make backup easy and data access easier.
You need a seperate licence for each copy of Windows on each PC you install - so you can't go out and buy 1 copy of XP and install on 2 PCs. Additionally, Retail versions are transferable between PCs (when you scrap one PC, you can put it on another), OEM copies are not (when you scrap/sell PC, you have to scrap/supply licence with it).
Outside of a domain, you have Computer Administrator accounts and Limited User accounts.
Thanks, everyone.
Could you still upgrade a peripheral, e.g. mobo or processor without having to scrap the licence? Also, as someone has suggested, is it perfectly feasible to have XP and Win2K PCs talking to each other, or at least sharing a router?
XP and W2K will happily coexist. Older versions will need tweaks to cooperate.
As to the OEM copy, and upgrades, gets very grey. If bought with a piece of hardware (only way to buy in retail channel), then that piece of hardware has to remain. If bought with PC, then upgrades are allowed as long as it can still be considered the same PC - clear as mud 
Thanks TB. 
I did think about a new system, but most seem to ship only with Vista and it's not that friendly from what I've heard.
Many IT people are lazy and can't be bothered to learn new tricks, hence moan about Vista. Vista is very user friendly, just takes a tiny bit of relearning in places. These same people hated XP when it came out, often calling the desktop Teletubbie land etc...