Accidentally Changed The Permanent Default For .exe Programs On Desktop

I was trying to download a trial of adobe photo shop to do my homework and i am on my moms laptop. i went to open the program and a dialog box popped up asking what program i wanted to use to open this file, (i didnt notice the checked box at the time stating it was going to use this option for all files associated with this extension!) and the only available program that was listed was WINDOWS MEDIA CENTER and i needed to use the program so i clicked on it.... wish i wouldnt have. it changed ALL my .exe programs ie; google,firefox,system restore and now anytime i try to run anything the media center comes up!!

I have seen on the web that several other people have done the same exact thing. I was freaking out cause this is not my computer and so i went to accounts and made a new admin desktop, which is what i am on now. i have found involves doing something on the actual desktop that is messed up. but i cannot open any executable files on that desktop ie;command prompt,so i cannot do anything on there.

i know there has to be some way to reverse what i did, i just need someone to walk me through the process so i dont screw the computer up any more than i already have, i am able to open system restore in the new desktop, i but dont think that will have any affect on the change i made let alone have an effect on the other desktop from this desktop. I also went to the place on the desktop to change the default for programs and .exe is not listed.