Formatting sometimes is a very essential thing. In case you installed some bad application in it and your mobile started acting weird. Or you want to free up the memory in C drive.
But N97 is a very advanced multimedia computer and it came with dozens of preinstalled goodies, some of which are very costly. So what are you supposed to do?
I faced the similar problem and was surprised to find that there was no article written about it anywhere, even the Nokia care people weren’t much helpful. 
So I took the risk and did it and now I can tell you how to do it.
There are two kinds of format options – Soft format and Hard format.
Before you do any of the formatting it’s good to back up your contacts, calendar and notes in PC suit or ovi suit and you can even sync then online to ovi site. Do not backup anything else, it messes up the settings. Assuming that you have backed up your data, lets proceed to the next step.
soft format – The soft format merely restores the factory settings and cleans up the C memory a bit. This can be used if you want to free up the c memory but isn’t much helpful if there’s some problem in your mobile because of virus or half baked applications. For the soft format go to - Settings-Phone-Phone mgmt.- and click on factory settings and enter your lock code (usually 12345). It’s done. Now restore or sync back your data. In this type of format you won’t be loosing any of your applications or games.
Hard Format – this one is a bit more sophisticated than the last one. I suggest that you first format the mass memory as it might have some apps installed in the mass memory that’s causing the problem. (Be warned here… You will lose all your data, so connect the N97 to the PC in mass memory mode and manually cut paste everything you need) For formatting the mass memory go to File Manager-Mass Memory and select Format Mass Memory. It takes some time so wait….
After it’s over switch off your N97 and hold three keys – Shift, Space & Back keys simultaneously (see image below) and press power button. Hold the keys till you get the screen to choose the country. It done. Now restore or sync back your data. After this your N97 will be as good as new.
Sadly in this type of format you’ll lose all your preinstalled apps and games and some of the apps are very hard to find. (I still haven’t got System Status back (help?) and lost marble maze game, which my mom likes very much but I can’t buy it because it’s costly) I contacted Nokia Care about it and they also confirmed that there was no way to get back the apps and games, whereas there is a nice page full of apps and games for Nokia 5800 Xpress Music . I tried to install some of those apps in N97 but unfortunately they were locked for Nokia 5800 and won’t run in N97 (Garr…) so do the hard format only if you or your N97 are in neck deep trouble. 
I had to buy Guitar Rock Tour again -
If you have any doubts, feel free to ask about it in the comments.
