A Mac is always a good option, but if your one of those annoying folk who believes anything other than Windows to be inferior junk, then convincing you would be difficult.
(And if you're on a wireless network, it's really nice.)
The best, of course, would be Linux, but I'm assuming you don't devote your life to complete knowledge of computers, so it's robably out of the question. If you know how to best use it, it's actually a good idea, but most people don't.
Therefore, for many, Windows is a good option; but don't get Vista. Until AT LEAST SP1, don't even think about Vista. There's too many kinks to work out. Whatever you get, whether 2000 (still a good option, if you're using SP4) or XP, don't use the basic version- use Professional. The advantages are too numerous to list.
Hardware! Make sure the lappy has at least 1 GB of RAM, and if you plan on eventually using Vista, consider 2 GB. Make sure the processor is relatively new: Intel and AMD both have high-quality 2 layer processors. (Unfortunately, as far as I know, Intel Quad Core isn't available for regular laptops; only servers use it.) Make sure it has a relatively large hard drive; obviously, portable hard drives partially remove this need, but it's more convenient to just start out well. Make sure it has a DVD-RW drive, capable of reading
and writing to all media. Find out what spped the rest of the network you're on is running at: if it's a T1 link, make sure the laptop's NIC (Network interface card) isn't stuck at 10Base T. Make sure it has at least one PCMCIA slot, and plenty of USB and FireWire ports.
If it's in you price range, buy a laptop by Alienware. And one for me, too.