Hi Ive noticed this being asked on the web alot so thought id post it here for all to use.
How to reset to my original settings on the Nokia N91 without deleting my files.
You can reset some of the settings to their original values by pressing the Menu key, and selecting Tools > Settings > Device settings > General > Orig. phone settings. To do this, you need the lock code. After resetting, the device may take a longer time to power on. Documents and files are not affected.