It's probably not too serious if you didn't accept the trojan's suggestion to allow it to infect your system...

First, try starting up in Safe Mode (keep tapping the F8 key as soon as you switch on, until the menu appears) and try running AVG from there. If you cannot access Safe Mode (it asks for a non-existent Administrator password), then things are a bit more serious.

If so, check out ComboFix on Bleeping Computer. If you can understand the instructions you'll be able to sort it, otherwise you need a local friendly face!