It's windows 2000 pro, with objectbar. Objectbar is fully customizable windows start bar replacement. It took finding a skin I liked and messing with the settings for about 2 hours to get it just right, but I'm loving it! If you google for ObjectBar, it'll come up. I'm also using windowblinds to make the windows look more like the objectbar colors and stuff, but I didn't show any of that stuff in the screenies. I recommend this to anyone.
Something else, if you feel like customization:
If you want something like Linux but want windows, check out litestep. It's free, and stuff. It's a fully programable/skinnable replacement for explorer.exe. Pretty cool, but I didn't feel like getting into detail with it. I mean this, though, you can make it exactly like Linux, but be able to use all the windows things. It's also faster than explorer.exe. I had it on last night, but took it off because I didn't feel like learning how to use it. I would seriously check that out. Now, this thing is powerful, so I'd only recommend it to people that are good with coding/etc.