Most of the time the prerequisite checker when installing SCOM 2007 is right that there’s a prerequisite not met to install the specific role or specific item of SCOM 2007.
However If you are 100% sure everything is there you can bypass the prerequisite checker by running the install with the following command:
MSIEXEC /i <path>\MOM.msi /qn /l*v D:\logs\MOMUpgrade.log PREREQ_COMPLETED=1
This is however NOT supported by Microsoft.
Note: in Windows Server 2008 always run commands in an elevated prompt.
This should be your last resort to get things going. Most of the time there’s indeed a prerequisite not met and therefore the checker is right.
If you want to double check your prerequisites you can find them here:
http://technet.microsoft.com/en-us/library/bb309428.aspx
A known issue with the prerequisites is that ASP.Net is not correctly detected. More info here: http://support.microsoft.com/kb/934759