I have a lot of simple pranks on here for you to enjoy. But if you are new to the whole pranking your friends business here are some of the most simple pranks that any one can do.
Desktop Copy: One of the most simple but effective pranks is the desktop copy. What you do is take a screen shot of your friends desktop. Most keyboards have a print screen key, located some where in the top right hand corner. After you have hit print screen you now have a copy of what ever was on the screen, in this case their desktop.
Open up paint, or photoshop and past the image (ctrl + v). Save the image as something simple and place it in your c: drive some where.
Next go into your desktop proporties by right clicking on your desktop and selecting properties. Under the desktop tab, click browse and select your newly made image.
Finally remove all the icons on their desktop. Copy them into a new folder and save it some where you will remember.
That is it. When they get back to their computer they will try to click on the icons but to no avail.
You can also hide there windows bar by right clicking on the bar, and unlocking and auto hiding. Then drag it to the top where they wont think to look for it.
M and N: If you have a friend who still types and looks at the keys. You know the one fingered type. Remove there M and N key and switch them. Simple easy and they will sure to be annoyed.
Stop Looking at Porn: This one can be bad so watch what you do. This is a prank for the simplest of computer users. Simply change their default web page to something bad.  I recommend either making your own web page that is chrome-less (no minimize, maximize or close)… Or finding a web page that already does this. This prank works best on people who still have msn.com as there home page. If they haven’t changed that, they wont know how to change it back.