I bought a Sandisk 4GB USB Micro flash drive. The U3 software that comes pre-installed on it will not allow you to access the drive until you put the password in. Some people hate the U3 software package, but I love it. It is easy to use, has a variety of free programs and utilities, and you can basically carry your own desktop around on the thing, while barring access to anyone without the password.
I do not think that U3 is useable on anything but what it's pre-installed on.
Still, my drive is 4GB, has a retractable USB key, it's black, and it's durable; mine has been through a complete cycle in the washing machine and still works without a problem.
If you want to restrict access to a computer, the best and most simple way is to hit it with a System password in the BIOS, then put a lock on your case/tower to prevent people from accessing the internals [you can reset BIOS settings and passwords with battery removal and a jumper setting]. What this will do is make it where people need a password to get anywhere beyond the BIOS screen; you won't even be able to see the Windows bootup screen, and won't even be able to work around this precaution by using another drive [be it USB, floppy, etc] without a password.