Your AVG serial number should have been sent to you when you paid for your subscription.
Check your received emails from AVG.
If you cannot find it, try contacting them and ask for it.
Even so, the current version is 9, so yours is outdated. You should download and install the new one.
Temporarily, you can use their free version, which can be downloaded from this link:
http://free.avg.com/Then see if you can upgrade using your older license.
Hope this helps.