Hard reset is a deep reset method ,equal to format and reinstalling the software in your PC. After a hard reset your phone will function more smoother. If you have lot of issues related the software installed in the phone ,and not getting some new features even after updated to latest firmware version ,just do a hard reset. There are short codes to do hard reset ,that dont work in S60 v5 devices as there is no similar key pad like old devices.
Warning : hard Reset will erase all of your data on Drive C: ,please take a full backup before doing hard reset.
1)How to do hard reset(Format) in Nokia XPress Music 5800 ?Press Green button ,red button,camera key and the power key together for 10 seconds,phone will restart ,if not switch off the phone and again hold all 4 keys for 10 seconds ,phone will restart and will ask the time and date settings like a new phone. Yes now you have done hard reset.
2)How to hard reset Nokia N97 ?
Press Caps shift , Space bar ,Delete button, Power On button together for 5 seconds. the phone will restart and Re set done . [ you may want to reset your N97 ,may be few times in a month ,as some times it does not start and keep the statup screen forever. If you cannot enter insdie the OS you cannot do soft reset ,in that case I used to do hard reset ,since I have a full backup always I am safe. ]