But then again that is not entirely correct.
As any DirectX release, DX10 is backwards compatible, which means that a PC running the latest DX10 runtimes can still run (in theory, at least, and unless there are other software or hardware incompatibilities) software titles using legacy versions of the libraries.
The opposite can be held true: Windows XP is not compatible with DirectX10.
Ouh, that could be true... (haven't thought of that ^^)
Thanks for the correction, Friday.

After all I have no idea of Vista (and at least, I want to have an apple before one Vista-pack gets under my roof).