Libre Office
An excellent Open-Source office suite, which is available for free (though donations are welcome).
This will open and edit all Microsoft Office formats correctly for most users – very complicated documents occasionally cause problems, but not often.
Note: If you set OpenOffice’s default save formats to Microsoft Office formats you don’t need to worry about incompatibilities with your friends, school or office.

AVG Antivirus
This is a free antivirus package. It, in my experience, is more reliable and better on detection and removal than most.
It, by default, updates daily if you have an internet connection.

The Gimp
This is a very comprehensive image-editing suite for Windows. It is open-source, and free. It uses a multiple windows approach, and has every feature almost any user might need.
Note: It’s as complicated as photoshop – don’t think you can just install it and create masterpieces.

Google Chrome
Chrome is usually faster than firefox provided you have lots of processors and RAM (one process per tab). If short on processors, try Firefox instead.

SuperAntiSpyware
A good free anti-virus scanner. Most anti-virus programs (commercial or free) don’t make a decent go of anti-spyware.