Encrypted Winrar file and Yahoo email

I have encrypted a file (in Word) and when I try to send it via Yahoo Mail I get a warning telling me that there is a virus…This is happening to a few friends of mine so I duplicated this problem, but I can’t find why this is happening…Are “rar” extensions not permitted in Yahoo mail? My word file had just 4 letters….My computer is completely cleaned and up-to-date. Could somebody help me?
Thanks in advance!!!!
jviso