Appdata is a subfolder in Vista within the Users folder. There's one for each user account you have established on the computer. "Appdata" stands for Application Data. In it is stored all the settings that the various programs that are installed on your PC need to function properly, like Favorites in Internet Explorer.
Appdata's normally a hidden file, so I suspect that at some point in time you decided to make it visible. The files in Appdata should not be altered or deleted - but you now know that.
AFAIK, all you can do is try to restore from backups - if you have any. A clean reinstall of Vista will require that you reinstall all your other programs and will not solve the issue you now have.
Should you decide to do a clean reinstall, make sure you have copied the C:\Users folder to CD/DVD. Also, copy all downloaded software (the installation files) and music/video files to disk as well; while Vista does make a backup of sorts, I'd rather make sure that I have the files stored elsewhere.
Hope this helps,
I have 48,000 files in appdata. my husband tried to delete some and now I have no favorites showing in explorer 8. I have windows vista. Can anyone help with either of these problems. my computer needless to say is super slow. We reinstalled windows vista to correct any problems but it didn't help. What happens if I install vista fresh. Do I loose all my files. Thank you for any help received.