Based on an entry under Windows 8 app certification requirements, it seems Microsoft will not be offering any PEGI 18+ games through the Windows digital store. According to the site, anything with a rating over PEGI 16 need not apply.
It's an odd decision for sure, and more odd is the wording of this particular requirement, which suggests that games rated ESRB Mature are safe. That's a big "maybe," of course, because the description advises against "adult content" as a whole.
5.1 Your app must not contain adult content, and metadata must be appropriate for everyone
Your app must not contain adult content, and metadata must be appropriate for everyone. Apps with a rating over PEGI 16, ESRB MATURE, or that contain content that would warrant such a rating, are not allowed.
Based on the first line, we'd assume that Mature games would also be excluded from the digital store. There is also the issue that "Mature", by ESRB standards, would encompass quite a range of content. Going by this, we might guess the North American side of things will be much more forgiving toward games seeking certification for the Windows 8 Marketplace, at least compared to Europe.
That's not to say you won't be able to play any PEGI 18+ titles on a Windows 8 machine. The regulation would only exclude such titles from being sold via the Windows 8 digital store. This move, however, does strike us as a tad unusual.