The most simple way to do this is to go to www.logmein.com
First on your computer go to logmein.com and create a username and download "logmein free", after you did this on your pc, go to your friend's pc and visit the site again and login under your the same username, you will be taken to "My Computers" On this screen on your right it says Add Computer. Click this and install the program. Now go back to your pc and sign in again to logmein, now under My Computers you should see your friends pc, click on it. After you are connected you should see Remote Control on teh left side, click on it and you should be able to see your friends desktop, you can control the whole pc from this program.
Good Luck, and hope this helps!