Hi,
Well, I have only done two XP MoBo replacements, and in both cases I had to format and reinstall. This did not surprise me given the way the MS anti-piracy mechanism works. The MoBo is too large a change (unless it is the same MoBo) and will almost certainly be detected.
I have added hard drives and RAM without XP batting an eyelid, but have had problems with modems, sound and video cards..............most of that could be put down to drivers though.
I think that aeallison discovered bugs in the beta release of XP.
The scenario that I envisage is that you load the XP OS and it creates an uninstall record. This record contains the hardware configuration (that is the logical error, as there is no good reason for it to do so, other than to remove XP drivers) On uninstallation, it detects that the hardware has changed and crashes. That is a design flaw as a second HDD does not install new drivers. It also implies that the uninstall record is not updated when new hardware is added.
The way it should work is uninstall what it has installed and warn you that there is stuff you will have to uninstall manually, if appropriate.
I must admit that I am surprised that it seemed to work for the first couple of boots.
My suggestion:
1. Try SDK's suggestion and uninstall AVG, although that might just be a coincidence.
2. Reformat and reinstall the OS
This is a pure guess, but I suspect that XP tolerated the anomalies until AVG started to try to connect to the net on its own (as it does)..........that probably broke something. Even if option #1 works, you will still have to go to option #2 if my suspicions are correct.
Good luck
