I have the exact same problem and i can tell you for sure that's Avast's fault. Or, to be precise, it's the updater's fault: once the boot has ended the updater starts and for some unknown reason it absolutely trashes the hdd - in, usually, over 2 minutes it manages to make more than 40,000 I/O Reads and over 20,000 I/O Writes. It uses the hdd so much that the entire OS (XP SP3 in my case) is absolutely frozen - nothing moves, nothing works (you can't even open the Task Manager).
No, it's not a bad config, i run a bare bones install of Avast - i disabled WebRep since i've discovered that most of its flags are obsolete and if there is a threat the Web Shield will kick in, the Software Updater and 2 other shields. No. it's not a XP fault - i have just the internet connection and the Opera browser in the autostart and i never have more than 18 processes running (15 at this moment).
No this is clearly caused by a bad coding in the updater. And i can even point when it has started - with the V7. But it has got worse and worse with every new version. At this moment it's so bad that i'm thinking of dumping Avast.