Old dell P4 2.4 has worked flawlessly for over two years (even with my wife and daughter using it too). I used to use it as my main computer in my studio - for recording, mixing, softsynths like arturias modular moog, plus games, internet, burning CD's and DVD's, etc.
My new Dell AMD Turion x2 with 32 bit vista home premium has worked amazingly well so far. I've only had it a few weeks, but it is fast as hell, and handles everything i throw at it. I was hesitant about Vista at first, but it seems to be fine - some of the features are kind of nice, like being able to see a little preview of a minimized program, or having it fan out your windows in 3d (which allows you to choose the desktop and minimizes everything else). Most of the problems people are having with it are trying to load it on machines that they shouldn't be.
Now, with all that said.
All premade machines are going to come loaded with two tons of garbage software. I prefer to wipe the hard drive completely out, and load from scratch - but if that isn't an option, then you can manually clean out even the worst factory install. It just takes a little time and patience. Getting a computer completely set up for use usually doesn't take me very long - if you don't know what you are doing, you might want to take it to someone who does.
The negative things you hear about dell tech support are pretty accurate - they outsource most of their support, and a lot of their 'techs' aren't much more knowledgeable than the average user (this is true for quite a few different computer companies though--dell just sells a f**kload of computers every year, so you are going to hear an inordinate number of complaints from anyone selling that much of anything).
Keep in mind though, that if you are calling tech support, then you can't figure it out yourself, and expecting someone to be able to troubleshoot your computer over the phone, when their only way of accomplishing anything is through you - who has already probably been the cause of the problem in the first place, and either won't admit what you did (which would save both them and you a bunch of time and aggravation) or refuse to listen to their directions, which while they might not always be the way YOU want to fix the problem - they can usually get you up and running (though it might result in your hard drive being wiped out and reinstalled to factory settings).
I've done tech support before, and when you have someone on the phone who calls screaming because their computer is screwed up, you know full well they are the one who did it, but they won't admit it - then they claim to know what the problem actually is (due to their 'mad computah skillz') and won't listen to your suggestions on how to troubleshoot or fix the issue, then they finally shuts the f**k up and listen to you, and they finally do what you tell them, and then it amazingly starts working again.