I know that a person with physical access to a computer can do almost anything to it. My main concern with my Windows 7 guest account is privacy and sandboxing. I don't want a guest user to have access to files on other user accounts, and I don't want him to download malware that affects more than just the guest account.
I'm using Truecrypt full system encryption, and my regular user and administrator accounts both have strong passwords. Permissions are set so that C:\Users\MyUser can only be accessed by MyUser. If I were to allow a guest to log onto my computer and turn my back for a few hours, is there any reasonable way he could break out of the guest account, get past file permissions, or install malware that affects other users?
I keep my computer up-to-date constantly, and from what I understand zero-day exploits are pretty rare. The only thing I can think of that a guest can do to get around his limitations is install software that takes advantage of an unpatched exploit in Windows or some 3rd party service that has super user privileges. So I assume I'm relatively safe allowing a guest to have access to my computer. Is this a false assumption?
Note: My primary concern is ignorant or irresponsible guests. If I know someone wants to hurt me, I won't let him use my PC.