If you need to carry a powerful calculator around all the time for either your work or studies then perhaps it's time you minimized your baggage by installing PocketCAS on your Pocket PC.
PocketCAS is a free scientific calculator that allows you to perform complex equations on your mobile device. In fact, it's just as powerful as even some of the most advanced pocket calculators on the marketing. Among its capabilities, PocketCAS enables you to perform symbolic and numeric integration and derivation of non-trivial functions, and to solve differential equations.
PocketCAS also boasts support for a huge range of linear algebra problems, like solving linear equation systems, multiplying and inverting matrices, finding eigenvalues (and eigenvectors, of course) and much more.
The PocketCAS environment is fully programmable and it contains a number of predefined keys for much quicker inputting.
Bear in mind that PocketCAS is designed to perform advanced mathematics, and if you just want a calculator for more simple equations, this one may confuse you a tad, especially as there is little help inside the program.
Nevertheless, PocketCAS makes for an excellent scientific calculator for Windows Mobile.