Windows loads stuff into your RAM for quick access, but it will of course clear the memory if you're in need of it. So that's why in task manager it will say you're using a lot of RAM but really it's just the win OS helping you out by allowing you to quickly open programs you often use. But this isn't what is causing your problem.
Some versions of windows will only allow you to use a certain amount of RAM. For example Win 7 basic vs Win 7 home premium, there's a difference I think. Also x32 vs x64 systems: To use more than 2GB(or 3 or 4, can't remember exact number) you need the x64 version of your win OS.
Make sure none of your memory sticks are broken. Check in windows to see that you have 8gb installed. You can check this in task manager or control panel. Open my computer/explorer and goto Control Panel\System and Security\System
Other than that you could have background programs running, like a virus or download manager or something. check task manager. But that wouldn't use 8gig of RAM i wouldn't think. So it's probably what you suspect, som e win 8 bug.
Also make sure that you system is using the dedicated graphics card, not the integrated one. If the system uses the integrated card then it will use RAM to use as its VRAM. Also Win8 uses a lot more memory(graphics) than win7. But still, it shouldn't use anywhere near 8gb.. so.. I don't know.
Edit 1:
Also read this thread:
http://www.tomshardware.co.uk/answers/id-1842726/memory-leak-windows.html
It talks about a program "Killer network manager" causing some mem problem. Apparently solved by updating drivers.
Another idea is to make sure you have all windows updates, and all other programs up to date.
Edit 2:
You might also find some use in these programs:
memtest86 for some memory testing.
You can also run Windows Memory Diagnostics.
Also Start->Run->type in "resmon", gives a more detailed view of memory usage than task manager.
You can also use CPU-Z for info on memory.
"You can also install and run RAMMap it gives a better display than resmon and has much more information to help solve the problem."
There could also be a problem with the paging file. Apparentyl having to small of a paging file can cause "out of memory" reports.
I refer you to this thread for more details on the above
http://www.tomshardware.co.uk/answers/id-2053154/memory-wiht-8gb.html