I want to host a website on a desktop computer running Ubuntu with a Windows virtual machine. I will give away the computer in exchange for a number of months of remote web hosting. I want to add some kind of lock (hardware or otherwise) so that the end users will have difficulty just reinstalling Windows and using the machine as they want, in contradiction to the contract.

Ideally, I'd want the machine to die if reinstallation of the OS is attempted. It doesn't have to be completely insurmountable, but it has to be difficult enough to prevent casual reinstallation. Perhaps on bootup the system can check whether certain files exist on the computer and refuse to boot if they do not. I don't know if this is possible, but maybe BIOS is password protected, and searches for files before boot up. The files it looks for could be date sensitive, i.e. require remote replacement on a schedule.