As the error says it seems JustCode installer does not find that VS 2008 is installed. This is a strange problem and you are the only one that has reported it. I guess it is something with your environment.
First, could you send us a log from the installer so we can investigate more:
run the installer like that:
JustCode.msi /l*v JustCode.log
and send us the JustCode.log file.
To fix your problem, I can suggest you reinstall Visual Studio 2008 and try again.
If you don't want to go through the pain of reinstalling VS, you can run the installer like that:
This should force the installer install for VS 2008 and it should work OK.
the Telerik team