The classic definition of computer security involves "CIA". No, not that CIA, but rather Confidentiality (keep it a secret), Integrity (keep it from being accidentally or intentionally changed), and Availability (keep it where you can get it). Snow Leopard doesn't get an "A", if you know what I mean.
Apparently, Snow Leopard (aka Mac OS X 10.6) has a habit of wiping out "home directories" when a user logs in to a Guest account following an upgrade from Apple's previous operating system: just Leopard (aka Mac OS X 10.5). Typically carrying the name of the user, the home directory includes all the standard user file folders, including Documents, Downloads, Music, Picture, and more.
Another user was hit just this weekend. "Nooooo!!! This morning I had access to Guest Account and than all my data were lost!!!" he wrote. "I had 250GB of data without backup and I lost everything: years and years of documents, pictures, video, music!!! Is it possible to recover something? Please help me!!!!"
It's good practice to back up your data. Here at the Borepatch secret lair, we use one of these. It's a Western Digital 1 TB network attached storage device. Backs up the Windows and Linux systems over the network.
Drag your home directory to the network box, and you have your stuff (pix, music, etc) if your Mac eats it. Or if you get malware on your Windows box. Having all your data is nice if you ever have to Nuke it from Orbit.