Eclipse defaults to the
Java executable in the PATH. On Windows, this is usually c:\windows\system32\java.exe. The JRE/JDK installation always copies java.exe to that directory. (Actually, thinking about it, I suspect that the java.exe is not even used, but rather Eclipse reads the registry to locate the jvm.dll to load. But the result is the same since the java.exe in system32 also reads the registry to locate jvm.dll.)
If you have multiple JDKs installed and want Eclipse to use a specific one, you can always add a -vm entry to the eclipse.ini file.