I've done it yet again, screwed up my computer to a whole new level.

half of my icons are a windows default picture and when i click them they become transparent. when i try and run my ani-virus programs the icon goes transparent as well. i go to the control panel to add or remove programs and i can't click it. my itunes doesn't open or work either. i believe it is from my younger brother trying to get hack codes and he entered a cmd code or whatever on the run part of start. help please! i don't want to loose everything.. and remember i can't download anything! it just comes up and says save, then there is a default icon of the program and it yet again becomes transparent when i click it. any help would be so appericated!!!!!!!!