Use as little software as possible.
Use software that does one thing well.
Do not use software that does many things poorly.
Use a plain text editor. Not a word processor (word), a plain text editor.
Do not use your text editor for tasks other than editing text.
Do not use software that’s unmaintained.
Pay for software/service that are worth paying for.
Thoroughly delete all traces of software that you no longer use.
Do not buy a desktop computer unless your daily computing needs include video/audio editing, 3D rendering, or some other hugely processor-intensive computing task. Buy a portable computer instead.
Use a Mac for personal computing.
Use a PC for if you want to game.
Build your own PC for gaming at least once. The experience is worth it.
If you use your computer for your career, then buy as large an external monitor you can afford.
Keep as much as possible in plain text. Not Word or Pages documents, plain text.
For tasks that plain text doesn’t fit, store documents in an open standard file format if possible.
Do not buy digital media crippled by DRM technologies unless your intention is to rent the content for a limited period of time.
This is my version of al3x's rules for computing happiness. Check out his version for a little more detail.