I am trying to get my first taste of Android development using Eclipse. I ran into this problem when trying to run Eclipse, having installed version 4.2 only minutes ago.
I have just solved the same issue upon setting up my Windows 8.1 PC. Exactly like @George Papatheodorou mentioned above (sorry I cannot add a comment), Eclipse and JRE must be both 64 bit or 32 bit.
However, it seems for Windows 8/8.1 environment, you are going to get 32-bit JRE by default (and I do not know where to change that default for the download), as explained here: http://java.com/en/download/faq/win8_faq.xml
I was using 64-bit Eclipse so there was a discrepancy. I then installed 32-bit Eclipse and everything works fine this time.
So before bothering changing any environment variables, check your JRE and Eclipse version.
Of course you can use 64-bit JRE with 64-bit Eclipse. Just make sure they match because Windows 8.1 will give you 32-bit by default.