Can I use my laptop as a portable hard drive?

I am a PhD student working out of multiple labs and offices. As a result I am constantly moving files around and getting very confused by the resulting file management. I have tried to hook up my laptop to my network but I can't see its files from other computers. I know there are USB dongles that allow you to move settings from XP to Vista, and ones that give you "control" of a desktop from a laptop. What I am wondering is if there is a USB 2 dongle that allows me to access the files in my laptop from my desktop as if it were a portable hard drive. Both use Windows Vista Home Premium.

Alternately, would it be smarter to just carry a portable hard drive with me everywhere as my primary data storage, and use the laptop's hard drive for software and backup only?