Why aside, there are always a few problems.
Vendors such as Compaq, HP, Dell, Gateway, etc.
tend to use specific software and drivers that are not included in the
retail or generic OEM versions. It is a very, very good idea to determine
what is needed beyond the retail/generic versions, and obtain, copy, or
whatever the needed stuff to a CD before you start doing anything to the
existing installed op system. This is particularly true of laptops. If you
have the ability (and software) to make an "Image" copy before you start,
that is also a good idea.