Hog4 PC Downgrade Issue

Hi,
I wanted to check something on an older showfile today so i un-installed Hog4PC 3.6.1 from my laptop.
Then tried to install v3.5.1 but could or the program wouldn't open?
The entire install process runs as expected and it looks like it has installed but nothing happens when i try to open the program?
I then re-installed v3.6.1 and everything works as expected?
Any suggestions or known issues with downgrading?

Thanks Cormac