Windows lightweight Math Software

I'm looking for lightweight, free, Windows, Math software.
Something I can put an expression and get an answer, or graph it.
I tried Euler, but it is quiet complicated and HUGE.
Basic needs:

1. Expression Based
2. Supports Variables
3. Support Functions, User defined and auto loaded.
4. Supports graphs, 2D. Not really needing 3D.
5. Supports History.

What do you use? What do you recommend?

As calculator wxMaxima (screenshots) with export to png and LaTeX syntax.

For playing with graphs I use GeoGebra with export to png, eps, pdf...

For some special cases Wolfram|Alpha.

And also MS Excel.

And also a normal calculator - is often faster than PC programs ;)

I'm also interested in answers to this questions!

There is a Power Calculator from MS, but has a lot to be desired.