There's nothing to panic about, but certainly black hats will be able to use this source, even if you cant rebuild Windows as a whole.

The way this will mainly be used is to analyze code in core dlls and search for exploitable bugs (such as buffer overruns). It's much easier (and much quicker) to find problems like this looking at high-level (and possibly well documented) C code versus looking at ASM code (possibly obfuscated) in a debugger or disassembler.

There is also the possibility that though Windows as a whole can't be rebuilt with this portion of code, certain DLLs may be able to rebuilt (maybe kernel, IO, or driver related).

If that's the case, the possibilities are endless!

Regards