NOTE that your issue is slightly, but significant different than that of the original post. There is a known bug with snow leopard that Apple is working to fix regarding guest accounts. See this
for more info on the issue.
The only work around if you need to use guest accounts, is to disable guest accounts, and create a "visitor" account as mentioned in the article. Only recovery method is from a backup if you used one (Time Machine?).
Also, and I mention this only because you indicate you are a new mac user, this issue from all reports only happens if you upgraded from Leopard to Snow Leopard with a particular configuration of accounts in Leopard. I tried to duplicate the problem and failed. So if you purchased your Mac with Snow Leopard and are having this problem make sure the Apple tech knows you did not upgrade from Leopard.