Only last week I had the exact same problem,only with Paint.Net. In the end I changed a setting in the registry .In Windows Explorer type in `regedit'(without the quotes).Then go to-

HKEY_CURRENT USER\Software\Microsoft\Current Version\Explorer\FileExtensions\zip\User Choice.

Then delete `User Choice' as this is what is redirecting the computer to open zip files with the wrong program. Then restart the PC.It worked great for me,changing the zip icons back to normal and opening them properly.
You can also go here-

http://itsvista.com/2007/01/itsvista-tip-16-using-vista-to-zip-and-unzip-files-and-folders/

and here-

http://www.winhelponline.com/articles/105/1/File-association-fixes-for-Windows-Vista.html
if you want to read up a bit first. I found these sites very helpful.
Good luck
Goonster