On Mac OS X, I create a symbolic link to the folder I'm trying to back up, and put it in the Dropbox folder (e.g. /Users/zachary/Dropbox/DocumentsBackup —> /Users/zachary/Documents) and that solves my problem.
No idea how to do it on Windows XP, though.
Best Answer
The equivalent of a symbolic link on Windows is the junction point. You can create junctions with the appropriately-named utility Junction from Mark Russinovich at Microsoft.
On Windows Vista and later, there also exists the command line utility
mklink
.