Reactivating avast

I just upgraded my computer OS from win98 to winXP Home edition. I use avast free home edition. Part of the upgrade process says to deactivate anti-virus software. I terminated (in hindsight I suppose pause was the correct procedure). Now I can’t get it reactivated where the icon is in the startup window in the bottom right.
If I double click on the avast icon on the desktop, go to the skin of avast, click the @ on the left a box comes up showing it is disabled. if I move the slider to standard or high and then close the box ( I don’t see anywhere to accept changes) when I reboot it is set to disabled. I have tried to find a chat in here similiar to my problem but I can’t find any.

I was perfectly happy with the previous settings of scanning etc but I can’t seem to get back to that status.
Any help would be appreciated.
Don

You need to uninstall avast completely, reboot and install it again on change of OS.