Installing from NAS

I have a NAS. I access it with admin account, but at every installation I try to start from it stops at about 25% installation done with an error of “is not accessible, file not found”. these are installations I have used many times before, so I know they are not corrupted. My NAS is connected, I try to “retry” but it doesnt work. (i tried XP SP3 and win7 , its the same). it happens only with installations I copied there from my hdd. the setups that have been downloaded directly from internet to the NAS work well. is it some kind of cheksum error?
any idea? thanks