You don't need the Cakewalk VST adapter with PE 6 at all. VSTs are native in that version already. I think vs 4 was the last one that needed the adapter.
AFAIK / IIRC Uninstalling and re-installing you should check that also the registry gets properly cleaned by uninstalling. I've never uninstalled SONAR so I can't say more about the details concerning that aspect.
I find it very hard to believe that W7 would be the cause of the problems, but if you have W8...? Then there's quite a few years passed from PE6.
PE 6 isn't that old, after all, and I believe I've seen posts by PE6/W7 users.
A damaged installation disc, as mentioned above, could be the cause.