Can't get %rootdir% to work

Hi all, I'm having trouble getting the variable %rootdir% to work, (C:\ is the system drive).

I have all programs which don't require installation in the directory 'C:\PortableApps\etc\etc', eg. 'C:\PortableApps\System\Registry\RegSeeker'.

The following don't work:

DetectFile=%rootdir%\PortableApps\System\Registry\RegSeeker\RegSeeker.exe

or

DetectFile=%rootdir%PortableApps\System\Registry\RegSeeker\RegSeeker.exe

I have to actually tell it to look at 'C:\PortableApps\System\Registry\RegSeeker\RegSeeker.exe' before it will recognise that RegSeeker is installed.

This seems out of sync with the other variables available.

Using XP Pro, BTW.

Thanks.