from several threads spawning since last week, it seems like the installer guy and the release guy (who does the packaging of the software / files) needs to be shot and fired. heck, maybe EA should hire me to do their installer software
i don't have the game, so i can only give my opinions based on what people are experiencing as well as my experience working with installshield...
it seems like they messed up their installer script. it's not their fault for not including a "backup plan" if installation fails in the middle -- most people working with installshield have had this problem, including me.
if you encounter problem where you cannot un/install the software, there are ways you can do to "trick" the installer to think that you are doing new install (instead of repair/upgrade). please note that this is not supported and i take no responsibilities if it fubar-ed your computer setup

backup your system before attempting this:
+ look for c:\program files\installshield installation information. it's a hidden directory. in there, there will be a bunch of directories with strange strings (the so-called GUID). one of them corresponds to the GUID of MOH:AA (which is 0dea94ed-915a-4834-a87e-388d012c8e02). delete this directory
+ in the registry setting, search for the same GUID and delete their occurences
+ based on the feedback from their support, in the registry, look for and delete "Medal of Honor" and "MOH"
after that, retry the installation. either of these will happen:
+ you will be able to (hopefully) successfully install the software, and everybody lives happily ever after
+ your installation won't be completed, and you will get the same unpack error. this will mean you got a defective disk (they didn't package the files correctly). take it back to the store and exchange it
+ you are presented with repair/modify/remove screen. if this happens, you still got leftover key(s) that has not been removed (this one is searched by the installer software to determine whether you have installed it previously). look for it and kill it, or ask EA's "support"

team to help you out
not much of help, but may be worth trying .... hopefully

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