I haven’t actually tried this, but I’ll be doing so when I return the trial N97. You’ll know it’s worked when you see the initial set up screen, asking country of origin. After you’ve backed up your data onto memory card/pc/ovi, turn your phone off and simultaneously press these four buttons circled in red. The traditional method won’t work as the N97 doesn’t have the old keypad. It’s also useful if you’re also unfortunate enough to have to send away your Nokia N97 for repair or exchange, or maybe in future you maybe selling it and don’t want your information/details/media stored onto your device for someone else to access. This wipes the entire phone memory and settings fresh to how it was (or should be) when you bought it. If you’re unfortunate enough to encounter a problem on your Nokia N97, a hard reset maybe what you need to do.
