How to avoid my friends using my personal computer.

Hi experts,
I have many important documents in my computer which I don't want to share with my friends. But my friends are insisting to use my computer. What should I do then?
How do i secure my desktop, can i create a new profile for them?
Securing my Windows 7 desktop could be fun and sure could have many ways with this latest OS
Any suggestion is welcomed.
Thank you.
