The first things to keep in mind is the license type. There are three basic license types available to consumers...retail, OEM, and upgrade. OEM is meant to be sold WITH systems. If you build your own system, then you can purchase an OEM license for Windows...but some stores will ONLY sell you an OEM license when you buy some of the key components of a new computer build (such as a processor or motherboard). An upgrade license techinically requires to you have a previous version of Windows. You should have ZERO trouble getting a retail license of Windows...you can purchase it at anytime and install it on any computer (once).

The second thing is Vista vs. XP. Microsoft officially no longer sells XP. You will likely still be able to get it in some stores, but availability will become more and more limited if not already limited. Vista, OTOH, should NOT be problem.

The only other issue could be conterfeitting of the OS. It is possible that fake copies of the OS are a problem in some areas and some measures might be taken to try to stop that problem. This could potentially lead to restrictions on how you can purchse the OS (i.e. maybe you can only purchase with a new system).