Apologies for butting in, but I had exactly the same problem.
Running version 4.8.1296 on a Thinkpad laptop, with XPpro, connecting via a 3G usb dongle, and updates have been fine for the last two months since original installation.
Then a few days ago it froze during the database update at 49152 the same as karrie. No matter how many times it reconnected to the server it failed at the same point.
I let it ride for a day two thinking the next update would cure it. but all subsequent attempts also stalled at 49152.
Also tried using the repair function, but that returned the corrupted file message as others found, and the only restore things to normal was a via a completely fresh installation. According to the Avast control panel, before the uninstall the last functioning version of the database was 081222.
Incidentally the installation on the home computer (which connects via dialup) has been updating as normal.
Still, its all working at the moment