Well I think the best computer software for a physicist, is clearly Woilfram Mathematica..

The newest version is Mathematica 6.0, so I offer it to everyone. It can be downloaded for free via torrent

Also if you want to write your own documents in latex, then download the Miktex compiler from

here. And then dowload the "editor" TeXnic center from

here.

There are also several latex tutorials for beginners, and not so beginners

here and

there.

If you are after some Numerical program, unlike mathematica which is very symbolic, then I can offer octave, which is totally free. It is basically a free version of Mathlab. You can download from

here. And there is a nice tutorial

here.

This is what comes to my mind for now