So maybe you don't have a lot of money to spend on software? Open Source Software is not just free in terms of price, but you have the freedom to do whatever you want with it. The software below is usually available for both Mac and Windows. If you don't have an operating system, you can even get that.
LibreOffice - office
suite including word processing, spreadsheet, presentation, database
firefox - web browser
GIMP - can't afford Photoshop?
Inkscape - vector drawing
InfraRecorder - CD/DVD burning
Handbrake - convert videos, including DVD extraction
WinFF - converts videos between formats and extracts audio from video
Audacity - audio editing
VLC media player - plays lots of different video and audio codecs
Celestia - star mapping software and explore the galaxy
7zip - like winzip, but way better (and free of course)
Blender - want to make your own 3D movie and become the next Pixar? (Warning: steep learning curve!)
Pidgin - instant messaging client allowing many accounts (Yahoo, MSN, AOL, etc)
Adium - instant messaging client for Mac OS X
KeePass - password storage
WinDirStat - analyzes storage space on your drive
Ubuntu - Free Operating System, (a Linux distribution) only for the brave who are basically ready to abandon Microsoft Windows. Another option is Cygwin, which provides a command line tools in Windows.