Hi
I think someone mentioned
www.evesham.com - my current pc was bought from Evesham about 5 years ago (and I've upgraded it myself with compoentnts over the years) - been great and their service policy is excellent if there is any problem. They were very competitive price-wise then, but I have noticed lately that they are getting rather expensive comparatively.
I've recently bought on behalf of my brother and a friend (neither of them very computer literate and wanting something that works 'right out of the box') from Dell after doing some research for them and their current pricing is very competitive, especially their Xmas specials - decent rig with a 15" TFT monitor for about £500. If you are ordering online you can customise the basic model for yourself. These have recently been purchased and have no problems installing and getting them up and running, however, no long term experience to report I'm afraid.
There are lots of others too obviously, but those are my two main experiences with bought out of the box as I generally build my own.
Hope that helps and Merry Xmas. Let us know how you get on!
p.s. if you are buying (especially online) read ALL the small print re warranty, return policy etc etc.
Biker