you need to install it everywhere you want to get access to the encrypted data. What two OS's are you trying to use though, and what boot manager?
I want it on windows os. XP/Windows 7.
Just for my Knowledge:- Can i install it on dual boot with linux?
As per my knowledge EEPC is not supported for Linux...
just install it in both OSs before you enable it. I'm fairly sure this is not an officially supported scenario though. There are too many variables, like partition layout, type etc.