I'd second Billfred suggestion to go with a new Macbook from Apple, as with
Parallels 3 and a copy of Windows XP, you will now be able to run everything from intense 3D gaming to Inventor and CAD on your Macbook. When you aren't working on writing code or designing robot parts, you can utilize iLife (which comes free with every Mac) for all your personal and family stuff.
Even with the baseline Macbook ($1099) + Parallels 3 ($79) + Windows XP ($150), you're looking at a price equivalent to a decent Windows PC laptop, but with the added bonus of Mac OS X and iLife.
I use both Windows and Mac computers every day, and they both have their pros and cons. But after using Vista for quite a while on various computers at school (I still don't like it), and seeing the new Leopard 10.5 operating system from Apple, I'm going to have to side with Apple. Even my parents are considering replacing their aging Windows computer with an iMac when Leopard comes out.
Here's some of the cool new features that are coming out in Leopard, Apple's latest version of Mac OS X set to come out in October:
http://www.apple.com/macosx/leopard/
* If you can, wait until October with Leopard is released. Otherwise if you can't wait until the fall, Apple usually has good deals now for back-to-school sales where you get a free iPod with any new Macbook or Macbook Pro. 