The KBE boot environment requires at least 496MB of RAM to properly deploy. If you have less RAM than is needed you can run into several problems including BSODs, errors that the RAM disk is too small, errors that drivers are not loading, or even a blank white windows on a blank white screen once KBE boots that locks up the system.
Just because your system has 512MB of RAM does not mean that the system has the full 512MB of RAM for system use. If you have shared video or audio RAM that takes the usable system RAM to less than 496MB you will have this problem. An example would be you have a tablet with 512MB of RAM, but 32MB of that is set for shared memory, that only leaves 480MB of RAM, so KBE boot will likely fail. Also, the more custom drivers or KBE driver packs you have loaded, the more RAM will be needed to properly boot to the KBE boot environment.