I have a dell, and i have to say that i have had problems with it, however the warranty is awesome. Every time I have had an issue it has been resolved within 3 days at the most. I could take this thing out and run over it and they would come fix it!
I have a Dell laptop and I'm very disappointed in it. No only does it look terrible, but it runs terrible. Brand new, too. I'm thinking about exchanging it. I am running my HP right now and I'm happy with it although it gets really hot.
I have an HP tablet PC at work and an HP desktop at home. Like them both really well. I recommend HP all of the time to friends and co-workers. I wouldn't hesitate in buying the Toshiba either. I would not even consider the Dell, from my experience they are junk. 90% of the machines I get asked to work on are Dell and they usually end up having some piece of hardware that needs replacing that the owner is going to have to pay for. Also, Dell's support is probably among the worst I have ever had to deal with. I have also seen Sony and Apple mentioned in the thread. Someone I work with recently bought a Sony Vio and is very pleased with it. I saw one of the new Apple laptops that dual boot Windows and Mac OS at a conference recently and they are quite impressive. Both Sony and Apple are also quite pricy though. All in all for the $500 - $600 range I would go with something HP or Compaq, they make some quality machines.