Assuming this is for a SCH C business.
Under Business Expenses - Other Common Business Expenses you have several places where this could be entered. Pick the one you feel is most appropriate for you.
Office Expenses - You can classify it as such for obvious reasons
Rental Expenses - You are basically "renting" the right to use the software from MicroSoft, since you have to pay for it on a recurring basis, even if that basis is only once a year.
Other Miscellaneous Expenses - If you feel the prior two sections are not appropriate.
