False positive?

Running avast free version. prog updated this evening. now unable to run any serif programs - PagePlusX5, PhotoPlusX5 etc as all now apparently infected with MSIL:CRYPT-NB(TRJ). Been happily running these progs along with avast for ages. now what is happening? Have uninstalled all serif progs, scanned the discs (clean) but when I try to re-install avast leaps in with this trojan message. how do I get round it?

many posts about this already… see virus and worms forum section