When windows 7 beta build expires in august, does that mean I will not be able to use it as from that date? -

That s exactly what that means.

CKM is right. Once the Beta expires Aug. 1, you will need to reinstall Vista if you performed an upgrade installlaton or wipe out the Windows 7 partition if you dual-booted. Jeff Windows Outreach Team

Yes, but you usually can downloaded it for a trial period - 1 or 2 months. Then you would have to buy.