The title may not be totally accurate. I want to know if I use my current harddrive in an all new system, Will I have to buy a new Windows 7 Installation disk? or not. It would be nice to save about 90 bucks
The answer is it depends. If you have an OEM copy of Windows, then there are no transfer rights and you'll have to get a new copy of Windows. If you have a retail copy, then you will want to be sure to prepare the OS for transfer by getting rid of the old drivers, but other than that you should be fine with little more than a reactivation.
And while it sounds like this is the plan, just to make sure, you can only use ONE copy of Windows on ONE system. Try and do more than that and one or both of them will be flagged as "non-genuine".
As cl-scott said no way around OEM, i heard you could do a direct copy to another drive but it would have to be in the same system =P. Or if your a broke man with little money, there are other ways around shelling out $90, i will say Windows is pricey. You got ubuntu thats free, if not theres other things free you just gotta put yourself out there. World wide web and P2P didn't just happen overnight. Good luck to you.