Linux is looking like a legitimate option.
Slackware based systems are the leanest of the various Linux variants....Slax is a live CD that is pretty lean...and I have been using Vector Linux pretty heavily,
It is superresponsive, shutsdown quick, comes up faster than most of the majors like Fedora, SUSE, etc.....with Firefox and some horsepower...is a great web browser.....
By the way what are your applications?