KeyExist() seems to work fine on HKCR on my win7 64 bit computer. Even with 4.53.

The following example returns a 1.

 Code:
? KeyExist("HKEY_CLASSES_ROOT\Installer\Patches\79EB7C9295ED2A736A78A2DD351249A8")

get $

So, although I do think you should upgrade to the newest version. I'm not convinced that the problem doesn't lie elsewhere.