It depends on which problem you are talking about, certainly not that of the title, that is down to the other AVs having not even detected the JS:ScriptIP-INF (TRJ) malware in your temp internet files, but avast did when installed.
The other problems you mentioned outside of that in the title:
"Also - since loading Avast yesterday my computer is sooooo incredibly slow. When starting this morning it will basically freeze up for bit (blue avast ball stops spinning) as if it's thinking....then it takes off again."
"my computer froze up & could not access anything."
They are almost certainly a factor, in the freezes, etc. as the AVs fight for control over the scanning of your system like two dogs fighting over a bone and you have three dogs fighting over that one bone.
1. Yes.
2. You also have Sygate, which is also a firewall.