Microsoft Word itself has nice math formatting.
Click on "insert" on the menu bar- then the "object" sub-menu, scroll down to "microsoft equation 3.0". If you've not ever used it before you might have to wait while Word loads it from memory, possibly even asking you to put the "Microsoft Word" cd in the drive- although that has never happened to me.
If, like me, you use it a lot, you will want to put the "equation" icon on your tool bar. Click on "tools", then "Customize". Click on the "Commands" tab, click on "insert" then scroll down the box until you find "equation"- drag it to your tool bar.