Q: I have successfully registered to use your avast! antivirus program, and I received the activation key without any problems. However, when I try to enter the key, the program says "Invalid license number". Why?

A: The activation key is valid. Try using the copy & paste method to enter it into avast! (watch the instructional video); Also, keep in mind that this number can't be used on any server operating system, and that you have to be logged in to Windows with administrators rights (to enter they key).
Finally, check if your system date is set correctly. The license key is valid for one year, so if your system date is set to a value when the key is expired, it becomes invalid. In that case, set the system date correctly and enter the same license key again.