It's just a guess, but your Safari preference list is probably read-only or corrupted. The plist is the file that stores general Safari settings, and I suggest you might want to make a backup of your current plist, and then delete it, and restart Safari again.
The plist can be found at
your home directory\Library\Preferences\com.apple.Safari.plist
So again :
- Quit Safari
- Backup com.apple.Safari.plist
- Delete com.apple.Safari.plist
- Restart Safari
If the problem could be duplicated on both Macs, it might be a plug-in that is causing the problem. Disable all plug-ins and re-enable them one by one to see if any is causing this symptom.
Found a thread on the same problem you have. Apparently it is one of the input managers (plug-ins) causing this symptom - in particular TabExpose.