Computer advice needed

Two prongs of computer advice needed.

1. My computer has gotten incredibly unstable -- it freezes constantly, the pointer freaks out and moves with random velocity, and tonight it developed the charming behavior of taking >60sec to respond to any button press. I am typing this now from safe mode, and am having no difficulties, which tells me that a large chunk of my problem is software. What should I do? It is a just-past-warranty Dell (how many of you are surprised?) running XP Service Pack 3. It has antivirus software (Norton?) with automatic updates, IIRC.
   
   
2. Obviously, it will soon be time for a new machine -- probably a laptop given all the travel I'm still doing. I will probably not get another Dell -- their service is (mostly) exemplary but their hardware just isn't. At the moment my main needs are word processing, spreadsheet, Internet, and some random proprietary programs. I am somewhat committed to Windows because of some software I used in grad school, but as that world fades my need for the software may fade, too. However, it would be nice to publish the dissertation first. I am very open to a suggestion that includes Windows XP as a second system, if such a thing exists.