Lock my account (administer privilege) when away for some period of time?

Suppose i am using my account in windows 7 that has administrative privilege. then my friend comes to my room and wants to use it. I surely can't deny him using but i surely don't want him to know my valuable info.
So is there any software that automatically changes the privilege to simple account without administrative privilege or can lock my other hard disk drive for short (for period that my friend is using my laptop) and again deactivate the setting and run with full privilege??
