If you REALLY want a laptop for under $400 then get the $399 ASUS laptop. It's very small, but has everything you need for Internet and typing up Word documents (if thats what you plan on doing with your laptop). One of my friends bought one and it fits in his backpack front pocket... it's sooo slim and small, as well as light weight. Battery lasts a long time too! If you want portability and aren't looking for a powerful machine, its worth it. Runs on Linux though.
http://www.staples.ca/ENG/Catalog/ca...fixedcode=W W
However, as previously mentioned, Celeron is
not meant for intensive programs, graphics, or anything that will require big computing power like gaming, etc. You will be able to surf the Internet (keeping online flash games to a minimum), Word Documents, listen to music (mp3's), the basics.
If you want to bump it up a notch and go for a bigger screen and more power, but have some fun apps to play with and stay organized, I recommend getting the basic MacBook from Apple. You can run both OSX and Windows on it, and Mac comes with iLife which has nice apps to get you started. Long battery life and really good for organizing. A lot of friends have them and are very happy.
If you want PC, I'd recommend a Dell, Sony, or Toshiba. I have a Sony VAIO and, while a bit more expensive, has lasted me over 4 years and is still an amazing machine. High end, slim, and reliable.