Here are some kits I've put together of graphics and sounds from different operating systems, mainly Windows. It doesn't have everything but hey, it still has quite a lot (especially in terms of Windows sounds, since I've been collecting those for a year and a half now). They can be used for a lot of things - making OS/UI/UX mockups, wallpapers, themes, even music based around OS sounds.
Quite a lot, I suppose. I've had to download quite a lot of ISOs to rip graphics and sounds from them. I also used websites to download various background packs. Around 50 people also contributed to this too, making the job easier.