The OS that you are getting is a seperate puchase from the kit?
There is no such thing as needing a new copy of Windows 7 that's what you are buying. If you later on change out your motherboard for a new one and you want to reload Windows that could be what someone was refering to.
I'm not sure that is entirely true either , I have a Windows XP OEM that I bought and when I went to change out the motherboard it wouldn't automaticly authorize the the Windows install and it gave a number to call and I had to manually over the phone enter in the authorization code that they gave me and it had to be done that way and there was no trouble and I actually did it a few more times as well.
So you are all set with the first install unless it's an upgrade version and then you will need a copy of Windows to verify that you are upgradeing but that can be any copy of Windows not just Win 7.