Just want to throw this out there.
All dell's are not alike...and the same can be said for every other brand.
For instance...Dell has 3 main lines of laptops. Inspirons are targetted towards home users and the consumer market. Vostro's are for small business, and Latitudes are designed for enterprise or large business customers. Inspirons are cheaper than Latitudes...and it shows in build quality, overall feel, durability, weight, performance, life span, etc.
Same with HP. The laptops you can buy at Wal-Mart or
Best Buy are going to be a significantly lower quality than the business-class machines. They don't have snazzy naming conventions to tell them apart though ;-)
Vista vs. XP....
Vista is more demanding on systems. Any new laptop should be able to run Vista just as well as XP. There are various tweaks/settings you can make to get rid of some of the more annoying Vista features (anyone interested, shoot me a PM). XP will no longer be sold on new computers as of 1 July, if memory serves. If you have a choice (not likely except from Dell via their small business, educational, or enterprise business units)....I'd probably steer you to XP. I personally run Vista on all my computers (work and home), but I'm not a typical user. XP is proven, most people are fluent in its use, and it just works.
My take on Macs....
Lemme preface this by saying I'm a PC user. I have been since 1990....the dark ages. DOS 5.0. Windows 3.0. I still do stuff command line. A few years back, I finally overcame my complete bias towarsd them (pretty much when OS X was released). The MacBook Pro's and the G5 series of computers are aesthetically beautiful, well engineered, and very stable machines...but they're not for everyone. For those of you that suggest running Windows on the mac hardware, I view that akin to driving a sports car to the grocery store. Sure, it'll work, you'll look cool, and feel awesome...but what's the point of spending all that extra money?
One more thing....
Accidental damage warranties. Not the Best Buy PSP, things like Dell CompleteCare or the Apple version that protect the laptop in case of spill, drop, whatever. These are on top of the regular warranty. You want that coverage. Seriously. ESPECIALLY in a college scenario. You can empty a full keg of beer into a running laptop, drop it out of a window, light it on fire, and they'll STILL replace the machine.