Upgraded RAM, Bootcamp doesn't recognize I have a Macbook 13' i5 early 2011, with Windows 8 64 bits and recently I bought new RAM kit from Crucial (8gb). After I installed the hardware, the windows OS doesn't recognize all 8GB, like the printscreen attached shows. Does anybody knows why?
Can you boot into Mac O/S? If so open up System Profiler, click on the memory option and see what the Mac is seeing. If it sees all 8GB then the issue is with windows and you might want to ask your question on superuser or another Windows forum.

If the mac side only sees the same 4GB then you may have an issue with the DIMM. It could be not seated properly, contacts could be dirty (clean the contacts with a pencil eraser and then rubbing alcohol) or the DIMM could be defective.

Another quick (and sometimes overused) trick would be to zap the PRAM:

Reboot and hold down COMMAND - OPTION - P - R (that's four keys) and let it bong a couple of times before releasing the keys and booting into Windows.