I was lucky in that the first computer we bought was fine from the start and we had no trouble with it. It was purchased from a company now out of business. After we'd had it some time, and before I became technically inclined, I had a local computer whizz-kid upgrade it for me and it went belly up. Had to reload Windows myself to get it going again, which took me the best part of a weekend.
Having been on a couple of courses, I can now build my own and the one at home is still going strong, if a little slow now, after two years. I'll be building a new one once we move house (if the move happens this time!). The one here in the office I built last year and the only problem I had was discovered to be faulty memory stick, which the parts supplier swiftly replaced.
My sister recently wanted to upgrade her computer, but as I didn't really have the time to do it for her, and she was desperate, she went to a local computer shop run by a couple of 'experts'. She was totally ripped off with the cost and had to go back several times before the computer would run properly. If I'd known she was going to go there I would have at least insisted that I go with her, but the first I knew was when she told me that she had the computer at home ready to set up.
I've heard nightmare tales from people who've bought from PC World, but then again they sell computers in their thousands, so they can't all be having trouble.
The moral is that you can get problems wherever you go. I know Dell are a reputable make, but I don't have any direct experience of them so I can't comment.
Try asking around locally if anyone can recommend someone to you. It is usually better to get a computer built to your own specifications, then you will get exactly what you want in the box rather than what the mass-produced ones have in them. The shop-bought ones usually fall short somewhere - that's why they appear so cheap, especially with printers and scanners thrown in, etc. If you have one built to your own specs, you can have a better graphics card installed or a better sound system, or whatever else you need.