Learn DOS Batch Files

Check out the following:
Doctor DOS Betamax
http://www.robvanderwoude.com/batchstart.php

Use Notepad++ for text editing

Notepad++

Some useful plugins

Directory for Notepad++ plugins: Plugin Central

File Switcher

NppMenuSearch

RunMe

Paint.NET: Alternative for Microsoft Paint

Paint.NET

Tabbed Browsing in Windows Explorer

QTTabBar is Windows Explorer add-on that allows you to navigate files and folders in a tabbed browser.....
http://qttabbar.wikidot.com/ or http://qttabbar.sourceforge.net/
(I have just tested on Windows 7, it works fine.)

Good calculators for programmers

1. SpeQ Mathematics
2. SpeedCrunch

P.S - I prefer SpeQ Mathematics.