I don't think that triggering on business day number of a month is a standard feature in QV management console (but I could be wrong).
Two possible workarounds came to my mind:
If you are firm in Windows scripting, write a little script that checks for the business day, if it's #5 of the month, trigger your task via an EDX call (there are some samples here in the forum of how to do this, it's quite easy using the command line tool). Let the windows scheduler run this little script every day.
Same idea, little different approach:
Create a small qvw file, check in the script the number of your business day with above suggested QV expression. Only proceed if the expression is true (or exit script).
Create a trigger in management console that let QV server execute this script each day (or 5. to 7. of a month).