Reader Matt Gohring has a laptop and a desktop, and he needs to keep his documents synchronised between them.
I'll give you a number of solutions to this problem, and let you pick the best for you.
The simplest and most seamless is to do your computing in the cloud. No, not on an airplane, but using Web-based applications that store your data on a server somewhere (and hopefully back up that server). For instance, Gmail and Google Calendar pretty much guarantee that I can access my e-mail, contacts, and appointments from anywhere.
The problem is that this requires you to learn new applications. I haven't switched to an online word processor, so I can't expect you to, either.
Another solution is to keep the documents you use regularly on a flash drive, and just not copy them to your hard drive (or copy them there for backup purposes, only).
Or you can use software designed to sync the two computers together. Here are a few recommendations:
Briefcase: You've already got this one. It comes with Windows XP and Vista. See Synchronize Files Easily With Windows' Briefcase for more information.
FreeFileSync: This free and open source program has some annoying quirks (I never got the filtering to work properly) but it's simple to use, it works, and the price is right.
Syncromagic: If you need something with more options and power, Gelsoft's program could be worth its US$40 registration fee (and you've got 30 days of free use to make up your mind). Options include shutting down the PC after finishing, filtering on date and/or file size, e-mail notifications, and FTP support. That later option allows you to upload your files to the Internet and download them later.