go for OEM, i dont know anyone who as ever used a call centre, and anyproblem the 'diagnose' can be done in half the time using the search button here.
as to the OS over the OS
you 'can' connect a hdd from a different system to a new one without formatting it, though it is very unstable because alot of the drivers are often for the other motherboard, btw, if u've got a dell, have a look on the case, there should be the serial number for windows, if it is OEM then no problem, just get a copy of windows which is the same version i.e. if it is a pre SP1 or SP2 then you will have to find the coresponding disk. check to see if the dell disk is OEM or specific recovery
i Installed a copy of XP on my desktop using the serial from an old laptop, worked fine for me