The Ultimate Scientific Calculator. From Microsoft. And It's Free.

Microsoft used to sell a program called Mathematics, which was aimed at students.  It was a very capable product, able to solve equations, draw graphs, do calculus, and much more.  Ideal for anyone involved with mathematics, science, engineering, physics, and so on.

So why do I say "used to sell"?  Have they stopped producing it?  No.  They've merely stopped selling it, and are now giving it away!  There's even a brand new version, 4,0, which was launched only last week.

You can get it from http://www.microsoft.com/education/products/student/math/default.aspx.  It runs on all versions of Windows from XP onwards, and is available in both 32- and 64-bit flavours.  The installer itself is an 18 MB download, although you'll also need .NET Framework 3.5 SP1, which is another 53 MB if you don't already have it.

If you're a student, or you need to do mathematical calculations in your work, then check out Microsoft Mathematics.  And if you use Word or OneNote a lot, there's even a version which works as an add-on in those programs too.