I've not run across Windows 7 machines touching our reg keys with explorer...so unless there's a good reason for it (such as another program trying to merge a registry backup or something), I'd investigate this machine to ensure it's not infected, rather than just excluding it.
This also begs the question..."why would explorer need to fiddle with our registry key?" As it's not something I've seen before on Windows 7...I'd be more inclined to believe something is on this box that's causing that...might be malicious, might not be.
Excluding explorer.exe, however, really isn't a good option, as you're undercutting SymProtect's ability to prevent tampering with files on the file structure. With explorer excluded, there's nothing to prevent someone from directly attacking the files on the file structure.
It might be worthwhile to run procmon and see what's calling explorer that, in turn, is trying to touch our registry key. At the very *minimum*, you should open a support ticket so we can check for malicious items.