Here is one way. It may or may not work depending on what your friend did. It is worth a try at least. Go to Start then to Programs, then Accessories, then System Tools, then click on System Restore. System Restore should have Restore Points in it. If there are Restore Points dated before your friend fixed your computer, you can try selecting one of them.
If that does not work, you may have to have your friend return and finish the job. Hopefully, he knows how to fix it.