Hello,
1. Locate the file "Adobe Photoshop CS5 Prefs.psp" on your computer and delete it or move it elsewhere. (I assume by doing this you may lose *** of your photoshop preferences but it will allow you to use photomerge and HDR so you choose)
2. On Windows this file can be found in the User folder under C:\**USER NAME**\AppData\Roaming\Adobe\Adobe Photoshop CS5\Adobe Photoshop CS5 Settings\ (Replace **USER NAME** with the Windows 7 account user name)
3. After deleting this file or temporarily moving it out of that folder you can then open Photoshop CS5 and use the Photomerge feature as much as you'd like.
4. As soon as you close Photoshop CS5 the program automatically makes a new Adobe Photoshop CS5 Prefs.psp file so when you try to reopen the program it will not work again.
P.S: This is a temporaly fix , if you have windows on 32bit , I recommend to use Photoshop CS 3/4 .