HCMON Driver fails to install (vSpehere Client 5.5 and 6.0)

I noticed that I can't install vSpehere Client 5.5 and 6.0 in the same workstation.

I can have clients 5.0,5.1 and 5.5 or 5.0,5.1 and 6.0 installed but not 5.5 and 6.0.

I always get "Failed to install the hcmon driver."

Windows 7 64bit