set_winver is broken, wine apparently wants the full build number, os version, and other info, not a simple one line change. E.g., diffing the registry between NT40 and WinXP mode (set in winecfg), gives:

[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion] -"CSDVersion"="Service Pack 3" -"CurrentBuildNumber"="2600" +"CSDVersion"="Service Pack 6a" +"CurrentBuildNumber"="1381" "CurrentType"="Uniprocessor Free" -"CurrentVersion"="5.1" +"CurrentVersion"="4.0" "RegisteredOrganization"="" "RegisteredOwner"="" "SystemRoot"="C:\windows"

[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Windows] -"CSDVersion"=dword:00000300 +"CSDVersion"=dword:00000600

I've started a more fleshed out implementation, just need to get all the right version numbers in there. I've got a flight later today and other things going on though, so may be tomorrow before it's done. Filing a bug as a reminder.

Fixed in r1947