I doubt I can write it off, but I just figured I'd check anyway. What if I took a class in it and needed the software to take the class?
I am by no means a tax professional, which is who you should probably ask, but from I understand about tax law: - If it is for a class or some other form of formal education, then yes, it can be written off. - If you just teaching yourself a new skill, then no, it can't be written off.
but like Bryan Denny said, if you are just learning, you can get the express edition for free.
I don't know about taxes on this issue, but you might be able to save some money!
Are you a student? Checkout the DreamSpark program from Microsoft and get it for free here. Additionally, your school might have a MSDN license or be able to get you a copy for cheaper than retail.
Alternatively, there is also the free express edition if you don't need the features of the professional edition.