McAfee GroupShield 5.0 For Exchange 5.5

Hi,
I'm having a problem reinstalling Mcafee groupshield 5.0SP1 on exchange 5.5, the story is that we had the groupshield installed on our exchange server but some problems were appearing, so one of our colleagues disided to remove it and install it again, when he did that he got a message "Installation ended prematurely because of an error" we follwed all the instructions listed in the mcafee website but nothing worked, so could somebody please give me a sloution to this problem?
appreciating your help, thanks