Most Netbooks aren't suitable to run "heavy" Operating Systems like Windows Vista, or XP. That's the reason why many users are installing a Linux distro onto their Netbook & some manufacturers like ASUS & Acer do offer their Netbooks with Linux pre-installed.
Tux Radar did test some popular Linux distros. The test Netbook was an Eee PC 701, featuring 4GB of internal storage, 512MB RAM and a 900MHz Celeron processor.