There are several free tax services out there that are web based. Double check you can't use them first. Express1040 is one I've used in the past, and it works fine on the Mac. The only thing I don't like about it, is that it made me double-dip into my education deduction, and I've had to pay back. But usually if your taxes aren't complicated it has no trouble. Most only cost money for State taxes, and that's typically under $50, and sometimes under $20.
If you can't find one that does all the forms you do:
https://discussions.apple.com/docs/DOC-2741
is how you run Windows on your Mac. Until April 1st, Windows XP updates are free. After then, you should go with Windows 7.