The AV should still work, just that you can't get updates until you register, takes about 30 seconds from the avastUI.
Avast wasn't the only one in this issue as you the user get a re-registration notification staring 14 days before expiry (if you had previously registered) or as Tech mentions before the 30 day trial expires.
If the avast just went off there is something else in the mix, but what that might be is debatable, but I doubt it has anything to do with the registration. Since you mention rootkits, there is a possibility that you were infected, but again no chance to investigate that.
I also can't see how this somehow caused a malfunction of 'all your software.'
I hope you will be happy with avg.