Okay, I have a fully patched Steam version of CKII, with all of the DLC. Oddly though, my checksum changes all the time. It switches between several different ones, and I keep seeing new ones. There's LFOE, NFOE, XGCB, and more. It's my understanding that it should stay the same until the version changes or DLC is added. Steam isn't installed in Program Files, and UAC is off, so it's not a UAC issue.