Price.
With the items you are listing you seem to be going for a very high spec machine.
My guess it is going to cost between �800 and �1000.
You can get a reasonable PC for between �450 and �500 from Dell, you do not need to pay that much.
I bought 2 PCs from Dell recently that cost about �425 each.
They were not top-of-the-range but were fine for general web browsing, email, office etc.
I could have added a better graphics card for about �50 pushing them to nearly �500, but they would have been fine for what you want.
If you wait for Vista you may have to pay more because the hardware requirments will be more, but a general purpose Windows XP machine should only cost you between �450 and �500.
Couple of other things:
Try to get a 19" TFT monitor.
Most PCs come with 17" as standard, but Dell often offer a 19" upgrqde for free, so look out for them.
In fact it should be said that Dell are ALWAYS offering special deals like more memory, larger hard disk, bigger screen etc so look out for good Dell deals.
They do offer a regular e-mail service.
Why not sign up for that and see what offers come out over the next few weeks.
I think that is about all for my marathon append, let me know if you need more help.
(I will probably think of something else in a minute)