I upgraded to Vista several years ago. Unfortunately my version of Mathcad would not work on Vista, so I installed virtual PC running XP just to use Mathcad. Since that time, I've been looking for a replacement, and I've found it, SMath Studio! I've been using it in serveral engineering classes that I teach and I am encouraging my students to use it as well. I've sent a donation via paypal and I will encourage my students to donate as well.
Keep up the good work!!!