Coverz has the right idea.
When your computer starts to load go into the BIOS settings, find the menu where you can set passwords, and you should be able to set like 3 passwords.
1. Password for the supervisor to acces the BIOS settings
2. Password for boot-up before windows loads (what you want)
3. Password for users when windows has loaded.
Once you have set the passwords you need to "exit and save" otherwise the settings will not occur. This should work.
If I have not understand someone let me know.
"Computers are considered female - As soon as you make a commitment to one, you find yourself spending half your paycheck on accessories for it."