The “N” variant of MyDoom is a repackaged version of “M”, which is currently rated as high risk by Secunia and Symantec. It appears to be as an email administrators message many times to trick users into opening ZIP and other attachment types.
MyDoom.N - new version of MyDoom.M
http://secunia.com/virus_information/10975/mydoom.n/
http://www.sarc.com/avcenter/venc/data/w32.mydoom.n@mm.html
http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=WORM_MYDOOM.N