These comments suggesting a Linux boot CD, or a Virtual Machine (VMWare , VirtualBox, etc) are all viable solutions if you trust your guest to stay within the environment you give them.
A VM, in my opinion, is really just useless, because the guest can switch away from it too easily and get at your main machine. Then perhaps become confused which browser is which, see your firefox on the desktop, double click and continue away... This is common with guests that are not too computer savvy....
Someone mentioned using a VM with a guest network and router firewall rules?? that's just more useless, the guest is sitting at your main machine. See the point above.
A linux boot CD is much better than a VM, with firewall rules to prevent this booted machine from accessing the local network, but any linux environment gives local access to local drives, so before you know it your (computer savvy guest) is browsing your local hard drive from your standard everyday system you use, and reading all your fine datas. Or if they are a reboot happy user (I've seen that, if the browser gets slow they power off) then that user may reboot when you're out of the room, and they may now boot into your main system and continue along, without you even knowing it, until much much later. You won't know this unless you are watching what they are doing every minute, and I am sure that won't go over well either.
The only way to go here is to have a separate guest network (hardwired or wifi or both) and have your guests BYOD. If you wish to be accommodating when they don't have their own device then you can give them a slow, cheap, small laptop from craigslist or something, and make them use that. Use any hard drive mirroring software to wipe and reinstall the Linux OS on it after they leave, or use a netboot to boot an image from a local server which you have a virgin copy of for the next user. As someone else already said, make sure it can access the printer, guests always want to print something.
I do the above. An old DELL Latitude D600 is the device for my guests. It has a 14" screen, 1 GB RAM, Pentium M 1.6Ghz, a 30GB hard drive, and dual boots Linux Mint or Windows XP so they have a choice if they care. The entire HDD is overwritten from a server image when they are done.
I say all this because I am the type of person that doesn't want anyone sitting at my local machine. I wish to give them full access, freedom to take their time and do what they want, without me watching guard over them to be sure they aren't reading anything of mine. I don't want them to start my Yahoo, or MSN , or read my email, my PC has years of financial data on it, local documents to my Condominium Corporation, letters to family, and the other 50% is ... well... we all know what the Internet is really for ;)