If you must build, a socket 754 Sempy with a mobo supporting onboard video will be more than sufficient (the Biostar 6100 mATX is excellent). Toss in a 60-80 gig HDD, a DVD burner (or CD-RW/DVD-rom if you can get it for cheap), a Forton 350W PSU, a gig of value ram, a copy of XP, and you will be fine. Get a case you can easily work in, and spend the rest on a nice 19" LCD. 
I agree that an on-sale Dell would probably be best, unless you really like building and/or predict that you will end up as customer support for them no matter who they buy from. The advice to set up Windows as an administrator and have all other users on a limited account is point-on. Set up their firewall (if they are not connected through a router) so that programs they normally use (and automatic updaters) will have access, and tell them to NEVER allow anything else through. Get Avast or AVG (both free), as they leave a small footprint and update themselves automatically. Banish IE and allow them to use Firefox only. Every so often, give the system a good scan. Ghost your initial setup in case things go wrong.