You can post your updates via your favorite scripting language. Zak likes Ruby but I prefer using AutoIt3.
You can post Twitter updates via Windows Cmd.exe and cURL.
curl --basic --user screen_name:password --data status=" $text_of_tweet " http://twitter.com/statuses/update.xml
Or from AutoIt with this line.
RunWait(@ComSpec & " /c " & 'curl --basic --user screen_name:password --data status="' & $text_of_tweet & '" http://twitter.com/statuses/update.xml', "", @SW_HIDE)
;$text_of_tweet = your update
Zak and I have written our own little twitter client programs to make tweeting easier.
Mine automatically saves all my postings to a file on my system for later reference and many other things to make life easier.
If you enjoy exploring how stuffs works, check out the Twitter API and cURL.