You said "How do I stop it from opening multiple sessions? Is there a switch somewhere?" and "it opens a version of the program with each file?"
I said "Only if SDI = 1."
You said "Both are set for 1." and "I have two pc's-One has SDI set for 1"
So your question was answered.... and you never mentioned anything about crashing until now.
So Windows itself crashes? Meaning what? BSOD?