In Windows Vista x64, drivers are required to be signed by someone holding a VeriSign code certificate or they won’t load. There is no way to (permanently) disable this signing even if you are Administrator. The F8 startup menu has an option to disable it, but you must select it every time you boot up. Microsoft’s claimed reason for this is that it prevents Trojans from installing kernel-mode rootkits. That is a load of crap.
In Win32, there is an API call called “MulDiv”: