How to know what someone did with my computer secretly?

Another way is to check history of modified files.

Click Search, check All Files and Folder, When Was it Modified, check Specify Dates and choose today
Leave Look in all Hard Drives option alone.

It will search all files modified today. You can see what is changed. A lot of it is system files got changed, but this way you can be very thorough. The Recent Items can be copied before they used, and recopy into Recent folder after they are done (after deleting all Recent items has just appeared during their work). That method is very troublesome but easy to do and circumvent checking Recent history.

One simple way: If files not deleted from Recent items. You can find it by click on Start/Recent items.
Anyway, your computer is personal so why would you borrow it to anyone?
If you have secret in your computer you must not share/borrow it to anyone else.

The best way to do that is install a Monitoring Software such as keylogger software. The keyloger software is an invisible keylogger that records all user activity and saves it to an encrypted log file. The log file can be stored as a text or web page. Use this keylogger to find out what is happening on your computer while you are away, maintain a backup of your typed data automatically or use it to monitor your kids....
Check out a lot of them here :)
http://download.cnet.com/windows/monitoring-software/?tag=bc
I recommend you get the Ardamax keylogger 7-day trial and worth to buy.

It's better to create a new account with normal user right or Guest and give that one this account. That user will not be able to install new software especially system software such as keylogger.
The first thing you should do now is backup your data. Then reinstall your computer.

Thanks Mr.Trung. Sometimes my roomate asks for borrowing my laptop.
By the way, I can't find "Recent items" anywhere. Could you please show me how to find it? I clicked start and searched but didn't see. I'm using Window 7. Thanks!:)