are you using a 64-bit system? or32-bit? u can find out by right clicking my computer left click properties and under syster theres system type , if its 32-bit u need to switch to 64 bit check that and we will see the index part after it
As h3sham said, you're most probably on a 32-bit system. Windows 32-bit only recognizes up to 3.5 gb of RAM which is why you can't see the actual numbers. The solution for this would be reinstalling a 64-bit version. However if that's not the case and you're on 64-bit OS, it may be a hardware failure as the WEI (Windows Experience Index) shouldn't change without any major updates.