Just remove all the folders named AndroidStudioPreview
On Windows:
Go to your User Folder - on Windows 7/8 this would be:
[SYSDRIVE]:\Users[your username] (ex. C:\Users\JohnDoe)
In this folder there should be a folder called .AndroidStudioPreview
On Mac OS X
Remove these files:
~/Library/Application Support/AndroidStudioPreview
~/Library/Caches/AndroidStudioPreview
~/Library/Logs/AndroidStudioPreview
~/Library/Preferences/AndroidStudioPreview