Linux n00b

Ok, so, the only OS that I've worked with is Windows and would to try Linux but since I've never worked with it I would like to know what's the easiest version of it, and if it's worth it. Also, suppose i would like to turn back to win (format the linux partition), can it be done without any changes with the file system (convert it)?

Please enlighten me.

PS:yes I know, I'm a n00b.

Some cd-runnable versions could be good for testing stuff and getting to know linux, like knoppix for example.