I am experiencing the exact same error with the most recent update. Using XPPro SP3.
In fact, I also experienced a similar error (I believe a different dll file was indicated) with the previous program update. Last time I was unable to repair or even uninstall the corrupted update, so I had to resort to using the avast uninstaller to remove the program. I then downloaded the new version and reinstalled from scratch, which was a huge time investment since I lost all my options.
Well, this time I once again tried to repair the update via "add or remove programs" with no luck. The repair program reports that the current version is 4.8.1229 and the new version is 4.8.1290, so I assume that the automatic update never completed successfully. Manual update via "add or remove" fails with a "There was an error while changing the product configuration" error notice.
I would go through the process of removal and installing the new update from scratch, but it appears that version 4.8.1290 has not yet made its way to CNET's download site, since it is reporting (and I confirmed after download) that it only has the previous version "Avast Home Edition 4.8.1229.80923" for download.
So I wait, with a non-functioning Avast, until someone gets their act together.