Windows 2000 source code would be the constructs or source that make up what we know as Windows 2000. You can't really buy it unless you think about it from the perspective of buying a Windows 2000 CD which in essence is the result of the source code. Right now there's a lot of talk about the source code being made available on the internet illegally for download, which some feel (not Microsoft) would pose great security risk to the operating system since vulnerabilities are easier to identify in source code as opposed to trial and error, possibly this is what you've been hearing about.
