I solved the problem by running the McAfee removal and cleanup utility downloaded from their website. It set the registry key back correct and probably fixed other future problems.
Download the removal tool from: http://download.mcafee.com/products/licensed/cust_support_patches/MCPR.exe
I downloaded and attempted to install the update from the pop-up window when I started the Incredimail e-mail client, from the Incredimail website and the update IncrediMail 2 Build 5175 from www.filehippo.com and
continually received the "Error 2738 Could not access VBScript run time for custom action" error dialog box on my HP laptop running the 64-bit version of Windows Vista. After I ried re-registering vbscript in the registry by typing cd %windir%\syswow64
into the command prompt and pressing the enter key, and continuing being unable to install the update,
running this removal tool from: http://download.mcafee.com/products/licensed/cust\_support\_patches/MCPR.exe solved the problem for me too.
Apparently, when I switched from Mcafee to Norton , the Mcafee uninstall was not very thorough. In addition, It speeded my system significantly.
The cause as explained in http://blogs.msdn.com/b/heaths/archive/2007/05/31/windows-installer-errors-2738-and-2739-with-script-custom-actions.aspx is that the Incredimail install/update executable uses Windows Script custom actions that are sometimes blocked by security software
(such as McAfee, in my case).
As some people have found, re-registering the runtime library vbscript.dll will fix the error, but that isn't always the solution even when you re-register
vbscript.dll, and you run regsvr32.exe in an elevated console on Windows Vista and newer with UAC enabled.