Guest desktop with no configuration permissions

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http://unix.stackexchange.com – I would like to set up a guest account on my Ubuntu so that anyone could log in (no password) and browse the Internet. But the guest user should not be able to modify desktop configuration (remove desktop icons, change theme, and so on). Is it possible? On one hand I need write permissions (browser cache), on the other - if write permissions to home dir are given, anything may get broken. Not mentioning running any malware by guest users... And there is another problem - if anyone would like to download anything from the Internet - they should be able to insert a pendrive and save there. On (HowTos)