I bought a pc from
Carrera as at the time the deal that had for the spec I wanted beat the likes of Dell etc. I was a bit wary of ordering from the net, but I have to say (touch wood) that I haven't had any problems to date (had it 6 months now). It comes with a 5 year warranty, 3 years on-site 2 rtb.
I had been down the building/upgrading your own path, which has been fun and you certainly learn a lot. But there comes a time when it's more cost effective to buy from new again.