I've encountered an issue updating AVG a couple of times now, so I thought it might be helpful to share the solution with other AVG free 8.0 users, to save you some time if you experience the same problem.
Here's the forum thread link, AVG 8.0 FAQ #1567 (I have paraphrased the answer below) and I've included the link to the utility file, to save you some time.

Just download deleteupdatefiles.exe onto your desktop, double click to run it - then try updating again. It took a split second to remove the corrupted files when I tried it, I barely caught a glimpse of the command window, and update went smoothly after that.

Follow their instructions to manually delete the corrupted files if the problem persists.

Hopefully you never need it!

Kimberly