If you're using Vista or Windows 7 you'd be better off putting your external runtimes outside the program files folder.

UAC that's included in the latest versions of Windows is a bit fussy about users putting files into the program files folder, sometimes it'll let you put them in then refuse you access to them or it'll just refuse to let you put them in to begin with & crash your machine (it did with me anyway).

My external runtimes sit in a folder called Runtimes in the root of my C: drive.

Windows 7 64Bit
Poser Pro 2010 SR1