Well, you could simply move all personal files into 1 big file and hide it. Simply hide it / make it invisible. Since he wont suspect that the file is invisible, he wont even try to find it.
1. Right click on your folder you want to disappear and select: Rename
2. Now, hold down the ALT key on your keyboard and press 0160 from the numpad while holding down.
3. Press enter to confirm it.
4. Now right click again on the folder and select Properties
5. Select Customize (last tab, don't know the exact translation)
6. Change the symbol to an invisible space.
Or you could use a program that locks / encrypts your folder/folders.
♣ Free Hide Folder. Hides the software and password protects the program to view it.
How to use it : http://www.ecuhq.com/tested-programs-how-to-use-f32/free-hide-folder-how-to-use-t184.htm
Edit : you could also put password in BIOS ( stage before Windows ). The only way of resetting BIOS password is by putting the motherboard battery out.
How to do it : http://www.ecuhq.com/security-f5/set-a-bios-password-t220.htm
Hope it helped.
More computer tricks, at : http://www.ecuhq.com