Why the '42000=unknown error when I have downloaded and paid for and received and entered my License key?
Can you give us some more information.
What is your OS ?
Have you had another AV installed, if so what and how did you remove it ?
When did the error crop up, what were you doing ?
Welcome to the forums, Steve! 
As David stated, we need more info to be able to help you.
I originally had Avast!4 Home.
I got a virus warning and tried to run the prog.
It would not run, I don’t know why.
I removed it via Control Panel/Add Remove.
I downloaded and paid for Avast!v.4.6 Professional.
I registered, recieved and entered the Release Liscense Number.
Currently the program opens and throws up the error message 42000=unknown error.
My operating system is XP Professional
Hope that helps,
Cheers
Futher to my last post, reg '42000=unknown error, I have received an email from chocholous@avast.com. He suggested I re-enter my new license key then go into 'Programm settimgs/…/Troubleshooting and there check 'Use XML storage instead of ODBC.
I did and have emailed him back to say the problem is stll there and nothing has changed.
Hope you can help.
Regards.
At which stage does the problem happen, exactly?
When you try to start a scan? Or immediately after running the main avast application (the skinned window)?
Could you post a screenshot of the message?
And has the program worked correctly BEFORE actually inserting the license key?
Thanks
Vlk