I finally solve my problem. After checking everything (registry, startup menu, services) I couldn't find any reference anywhere to a start of any blackberry software.
Since I was desperate, I just renamed the "Research in motion" directory in "Program files" and "Common Files" and restarted my computer. After my computer was fully started, I rename the directory back to normal and launch the install, and here it goes, I didn't get the message that the desktop manager is running and I manage to install the OS.
I am just still sceptical on how the blackberry software are started since there is no reference anywhere.
Thanks for your help guys.