How do I change a disk partition map scheme from MBR to GUID Partitions Table? I just finished the huge download of Mac OS X Lion, started the installer, and now I have a problem. The installer says that I must have the GUID-Partitions Table Scheme.

The problem is that when I go into the disk utility, I can't change the format anymore, because I have Master Boot Record as type.

Can anyone help me, please?
Ok thanks for your help, but I solved the problem on my way, here's what I've done:


Backup everything important to an external Harddrive
Inserted the Mac Installation Disc and ruined there the disc utility app.
I removed everything from my HD and it was GUID-Partioned.
Now I just installed Mac Os X 10.6.2 and updated to 10.6.8
Now I could run the installer of Mac Os X Lion without any problems and as a benefit of the whole thing, I started with a brand new Mac Os X Installation, without any trash in the file system.