Sorry if this is in the wrong section

I want to use a validation rule on the text which a formula produces. For example I am using an IF statement:
=IF((B6="AMD")*AND(B10<>"Pentium"),"Bad","Fine")
I would like a validation rule appear when the word "Bad" appears, but when ever i try to do this by creating a validation rule which will only allow a word which is 4 letters long(for Fine) it doest appear when the text field is "Bad", what am I doing wrong?
Thanks for any assistance.