You need a license key. The USB (or disk etc) can be used to install Windows limitless times, but the activation aspect is where issues arise.
A license key is good for one system only. If it's OEM, it can't be removed from one and installed on another.
If it's a retail key, you can move it - but only have one system active at any one time.
There have been known cases of a license key activating more than 1 setup (sounds like you have had that 'luck') but it violates the EULA and your copy of Windows is likely to turn 'not genuine' on at least one.
Typical scenarios (when not activated) are losing your wallpaper - it'll turn black, with a watermark in the bottom (right?) that states something to the effect of "Windows is not genuine, activate now".
As for updates on non-genuine Windows, I'm not sure. You're entitled to updates for a 30 day 'trial' period I think, but beyond that, MS have no obligation to provide you with any form of updates - critical or otherwise.
Do your installations of Windows show as 'activated' in 'System'?