Have you encounter problem installing application to your XM5800 where it prompt "certificate expired" or something, this happen to every application you tried to install. The only way to resolve this is by reset your phone back to factory default setting, so how are you going to do that?
I’ll provide the steps as follow and there is also a lot of guide available on the net and I just create my own for my reference as well. Rule of thumb before doing any factory reset or formatting, please do backup all your important data from your phone via Nokia pc suite and also remove your memory card from the phone slot before resetting.
1) Remove the memory card from the phone slot, press the power button and select "Remove E:" and on your left hand bottom there is where your memory card located. Remove the slot cover and press the memory card as this will eject the memory card out.
2) Now press on your dialer "*#7370#" this will prompt you "Restore all original phone settings? This will restart" and press "Yes" to continue.
3) Your phone will be reset and restart with original setting where you need to set some basic setting like time and date, language and so on…
4) You can plug back the memory card into the memory slot then you can install your favorite application without problem.
The steps is quite easy and only problem you need to backup and restore all the data back, hopefully this problem does not happen all the times. Since after you have reset and not install much application, you can consider to upgrade the firmware as well if you don’t have the latest firmware. To check the firmware version on your phone, press "*#0000#" and you can see something like below:-
If your version is lower than the image above, you can update by pressing the "Options" button and press "Check for updates" this will check for updates and download the latest version into your phone. Then you have option to upgrade the existing version and will restart your phone. I did encounter that i need to power off and on few times to have successfully upgrade the firmware. Alternative you can download the firmware updater from Nokia website and upgrade via Nokia application updater using USB connection.