Have a look around your local Charity Shops, there are several in my location and they often have used desktops, going very cheap. I bought one just a few weeks ago, played for it a while, connected it on-line, updated it as much as I could for free - just for fun, then gave it away to another charity shop. Also see plenty of scanners and printers in the same shops. Prices - £20 and under, for working, or mostly working PC's and peripherals.
I wouldn't buy from a charity shop or any other non-business source unless you are confident you can check the safety of the equipment - this goes for PCs and even more so for the cheaper CRT monitors. The cold cathode strip lights used in scanners on their own have enough voltage to send you to 'another place' and I don't mean the house of lords.
(Most charity shops in our area won't take such equipment as they can't legally sell it!)
I'd suggest either the new netbooks - purpose-built for email and surfing or local shops but beware - if any 2nd hand mains kit including PSUs hasn't got a current test label or waranty the chances are it hasn't been tested.
Jim_F, an excellent guide, most of the contents of which I have been aware of for many years - as far as I know all our local charity shops are also aware of the legalities, labelling their electrical items accordingly. We have locally, Help-the-Aged (or Age Concern - I am always getting the two mixed-up, old age I guess), Salvation Army shop, the Ark (animal charity) and a specialist Cat charity shop. So I have plenty of choices of buying electrical goods legally!