Ok, heres a bit of input, hopefully of some use.
consider these points:
Almost anyone can build a pc these days (thats not to say they will do it as well as it can be done, and it is also not to say they will ocrrectly install the best drivers etc- xp/vista are good at installing microsoft drivers)
A "high spec" (wich means loads of high numbers that actually mean nothing to a non technical person but sound good) machine can be had for a few hundred quid off the shelf.
An el cheapo 17" tft can be had for just under £100
XP can be had for about £70 ish or so now, as can vista (ok give or take a bit)
A cheapo pc case can be had for £25 with an equally cheapo PSU
so £195 so far
A cheapo board with onboard everything, about £35 (PLEASE dont go even cheaper and get a jetway or pc chips board, all they are good for is the bin)
CPU - celeron/sempron 3000 about £35 - the Ladas of CPU's
HDD 200 gig about £55 ish
writer, about £25
cheapo kb/mouse £10
cheapo speakers £6 if the monitor doesnt have them
floppy drive £5 (I ALWAYS fit them)
thats about another £100 which brings the total to about £300
Thats going off approximate decent retail prices off the top of my head, could be a few £ either way on each item.
so £300, then theres your time for building it, installing windows, drivers etc.
So that puts the selling price at about £400 to make it even worthwile thinking about it. Then if you are posting it theres postage, if you are delivering then there is your fuel and time, which of course you may offset with a setup charge.
You will have noticed that I use the word "cheapo" a lot, personally I'd either do it properly or not at all, unless I'm 10000% sure that all the user wants is basic pc use then I simply wont go there. I lost out on a tender recently for a small business (I actually do regular work for their other company and thought I had it in the bag) They wanted 30 vista premium (ok he'd been reading mags lol) ready pc's, server, comms, etc. Anyways I quoted for Decent base units, 939 X2's, decent vga cards etc, gig ram etc, . I didnt get it, another company did, they now have 30 celeron D's with 256 ram and onboard everything, of course they are FAST now! well they will be with nothing installed lol, when they come to upgrade to vista later this year then the fun will start. My point is, COMEBACK....
So you build a cheap pc, fine no probs, "its only for the kids" (Forgot to put antivirus on my list above) Then they come to do their homework, a typical phone convo might go like this:
Them: "That pc you sold me doesnt have windows"
You : "????"
Them: "It has no microsoft windows"
You: "Theres a problem with the pc?"
Them "Yes, theres no word and stuff"
You: "Microsoft office XP currently retails at about £300, but I can get it for about £200, you might even qualify for the student/teacher license thats even cheaper"
Them "I just spent £400 on a pc and theres no way I'm paying that for something that should have been on anyway"
..... and so on
Then when you sort that out:
Them:"Just bought HALO 5 and Halflife 4 for the pc but its really so jerky its unplayable"
You :[insert your thoughts here] "But you wanted a basic pc"
Them:"Yeah but I want it to be able to do simple stuff like play games"
I think you see my point. As you may have gathered, I am somewhat reluctant to build basic pc's. If someone wants a machine for a particular job, such as video editing then I'l be only too happy to do one, but again it wont be cheap because A

I want a machine that will do the job, and B

Whilst
I Know that just because a celeron D cpu has "More mega hurts" than a code duo 6800 doesnt mean they will know the difference between a £30 CPU and a £600 cpu that has "Less Mega hurts".
Not a lot of positives there I'm afraid, but at least my input might give you some things to consider, dont think short term, dont do cheap crap for a few £ profit, do it properly please ! (Not implying you wouldnt of course!!)
Regards