I have several folders enabled for sharing on my home Wi-Fi network. Is there some easy way to "unshare" these folders when using a public hotspot?

Good question. Security at public hotspots is a big issue, since most have no encryption or other protection. If you are using a VPN when you connect at a hotspot, such as JiWire SpotLock, there is no need to worry. All data flowing to and from your computer will be routed through a safe tunnel. Without VPN, you should indeed make sure that hackers cannot access your machine through file sharing.

There are two basic options for this. First, you could turn off file sharing altogether, in which case you'll need to turn it back on again when you get home. This is the safest method, but a potential hassle. Second, you can configure sharing to restrict access to only specially authorized users, and turn off any "guest" accounts. The latter requires that you disable Simple File Sharing (on by default in Windows XP), and then configure permissions for folders you want to share.

Microsoft has instructions for configuring file sharing in Windows XP here and here.

Mac users should also make sure they have set restrictive permissions for file sharing before connecting at hotspots (including turning off any guest accounts without passwords), or simply turn off file sharing altogether in the Sharing System Preferences pane.