The most imp thing before hard reseting your N97 : Take a full backup.

The hard reset completely erases everything you have on the internal memory (including settings), but doesnt touch anything on the mass memory. So if you want to format your mass memory, go to Applications -> Office -> File Manager then enter your mass memory. Select Options->Format Mass Memory.

Switch off your N97 and press these four buttons together for few seconds (till you get a white screen on your phone). If done correctly, the first screen you will get is ‘Select your country’.

Power Button + Space Bar + Caps Shift Bar + Delete button

Dont forget to backup all your settings and data before hard reseting your N97.