Avast doesn’t have to be reinstalled after applying a service pack.
The message ladybug was seeing only appears when taking the following upgrade paths:
- Windows NT/2000 → Windows XP
- Windows 9x / ME-> Windows NT/2000/XP
If you really haven’t upgraded your OS as above, then maybe the avast setup program was originally run in ‘compatibility mode’ (a feature of WinXP that allows you to pretend you’re running a different OS).
Cheers
Vlk