There are many reasons for you to use genuine copy of Windows 7 instead of Beta version of Windows 7. If you are using beta version, it can crash or expire at any time. You can not get updates and inbuilt antivirus updates in Beta version. If you are going to use Windows 7 for offline purpose only, you can continue with beta version, because there will not be virus threat from internet.
If you will be found using pirated copy of Windows 7 in office, Microsoft can sue you. For official purpose and for more security you should always use original copy. Future upgradation and support will also be available for Genuine Windows 7 users.
You can purchase Original Windows 7 at any big computer store in your area.