The best way to lock down a Windows machine so that people don’t install a bunch of crunge onto it is to only allow people access to the machine as limited users — you’ll create a new account in the Control Panel’s User Accounts section by clicking on “Create a new account” under Pick a Task…, choosing a name for the new account, and then selecting “limited”.

With this type of account, your guests will be able to access the Internet and do limited things, but won’t be able to install applications without your administrator password.

To further restrict what people can do with the computer use the Windows Group Policy Editor. As Administrator, click Start>>Run and type in gpedit.msc - this will allow you to further lock down the machine.