guys I wanna make a pseudo-OS in the browser

why? Imagine you had "try Debian" or "try Ubuntu" sites like you have for tryerlang and other programming languages
so my question is, can javascript write and read files from the user? even if only limited to a folder like %appdata%
or will I have to use something like Flash?

I'm confused. Is OP asking how to embed an SSH client into the browser? That would be the equivalent of all those shitty `tryfoola.ng' sites. Is he asking how to easily give windows users a way to try new operating systems? What he's looking for was pretty much already been done with Wubi, and it doesn't rely on a fucking browser.