1. burn all your important documents to a cd
OR transfer them to an external USB hard drive
2. Make sure you have a drivers cd so you can get basic hardware and the internet going. if not, go to the individual device manufacturer websites (say nvidia, ati, gigabyte, sis, intel, etc) and download the latest (working) version of the drivers you need. make a drivers CD out of that.
3. Stick in the OS install CD (windows, linux, dos, what have you) and reboot the computer. Enter your bios and set boot order so that CD comes before HDD. Save and exit.
4. Follow on-screen instructions.
5. Once installed and rebooted. Insert drivers CD and get busy. reboot accordingly.
6. bring all your backed up data/files from cd/usb drive
7. get on the internet adn download all apps and other junk you need.
my personal recommendations for apps (for windows):
Internet : Firefox (browser), Thunderbird (email), Pidgin (IM), BitComet (bit torrent), Frostwire (p2p)
Security : Avast! Home (av), Spybot S&D (spyware), Zonealarm (firewall)
Media : Foobar2000 (audio player), Klite Codec Pack (codecs), Infrarecorder (cd burner), Media Player Classic (video player)
Office : OpenOffice2
*note : all of the above are either open source or otherwise FREE (as in beer, many as in speech) applications