It is my understanding that the OEM versions of the operating systems sold on places like newegg are good for only one install and are targeted toward system builders. My question is, if I installed the OS after my build and wanted to format my HDD later, is there any way for me to have access to the OS still? Or will I have to buy another copy?
OEM versions are tied to the original system (motherboard) your install it on. You can install it on that system as many times as necessary, though I believe after a few installs, it may require a phone call to Microsoft for activation.
In short, so long as you reinstall on the same system (motherboard), you do NOT need to purchase a new copy.