If you have Windows XP or Vista, try using System Restore. For XP go to the Start menu > All Programs > Accessories > System Tools > System Restore. Choose a date prior to when your sister deleted files. The thing about System Restore is you can always undo a restore point and go back to the original setup or try another point.
If this fails to bring things back to normal, you will have to reinstall any missing items like the sound card driver as BlameCanada has suggested. However, I would like to add that if the sound card is integrated into the motherboard (onboard sound), I would go to the motherboard manufacturer's website for that driver if you don't have the motherboard CD.
Since you made reference to the Recycle Bin, it sounds like your sister tried to delete programs by simply deleting the folders and files. That is a good way to screw up the system. You should uninstall Programs using Add and Remove Programs in the Control Panel. Alternately, if the program has an uninstall feature shown in the Start menu, you can click on that. There are other technical ways to get rid of programs but these are the easiest and least likely to get you into trouble.