How do I change a disk partition map scheme from MBR to GUID Partitions Table?

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

1. Backup everything important to an external Harddrive
2. Inserted the Mac Installation Disc and ruined there the disc utility app.
3. I removed everything from my HD and it was GUID-Partioned.
4. Now I just installed Mac Os X 10.6.2 and updated to 10.6.8
5. 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.

search: http://www.google.ee/search?q=convert+mbr+to+guid&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

http://www.insanelymac.com/forum/lofiversion/index.php/t183105.html claims that http://www.coriolis-systems.com/iPartition.php is able to do MBR -> GPT conversion.

Then there's supposedly some not-for-faint-of-heart options mentioned in that thread, which IMHO judging from the tone of the original question, are not safe enough to use. At least make a whole disk backup first.