Most of the Nokia 5800 Express Music phones that you can buy today come with a nice (!) firmware bug that prevents you from installing any application on them: each try fails with a "Certificate Expired" error message. Here is how to solve the problem.

First you need to install the Nokia PC-Suite software that comes in your device package (or get the latest one from Nokia).

Then:

1. Backup all your data on your phone using PC-Suite (not needed if your device is brand new and you've got no personal data on it).
2. Check your firmware version (key sequence *#0000#). If your firmware is earlier than v11.0.008, it would be wise to update your phone (not sure this is needed but this is how I solved this).
3. Once your firmware is updated, reset it (key sequense *#7370#, device lock code should be 12345).
4. On reboot, re-enter the time and details of your location (no need to specify Honk-Kong locale or any other specific stuff as found on the web).
5. Here you are!: you should now be able to install application again (don't forget to copy your personal data back on phone if you have done this at first step).