Windows XP Pro:
Microsoft OneNote for notes.
Microsoft Visio for technical drawings.
Microsoft Visual Studio for programming.
Visual Assist for Visual Studio for improved IDE functions.
Intel VTune for profiling.
NUnit for .Net unit testing.
Scite text editor for scripting and XML.
WinSplit Revolution for managing my desktop windows.
Beyond Compare for comparing files and folders.
Cygwin for finding things with grep.
Paint.net for converting or [...]