Your problem is somewhat different to the above issue as it isn't for the current/recent version/s of avast.
Can you be a little more specific on this error as "access denied" isn't something I'm familiar with in relation to updates. A screen shot of the error window might help.
Support for avast 4.8 really is on its last legs, there have been several end of support dates, which somehow have got extended, the last was the end of last year. Now we are almost in to March/Spring 2013 and I don't know if that support (virus definitions) is still running.
What is your operating system (win2k, XP and later) ?
If it can use later versions of avast you should do so as there are very few people in the forums that have even used 4.8 much less remember its settings.