It is not installed, just running (and is holding the blockchain up to date). Actually I'm running a "full node".
If you - by installed - define a software which has entries written into the windows registry.. then alright, it is not installed.
You are still able to use armory.
I don't trust Electrum. Is it as save as Armory is?
Actually, both wallets are as safe as the computer they are running on is.
On an offline computer, both are perfectly fine.
On an online computer, it depends on whether your wallet is password protected (can be done in armory and electrum).
Electrum can definitely be regarded as a safe wallet. The downside of electrum is that the electrum-server you are connecting with can learn your habits and addresses, therefore compromising privacy.
Note that this doesn't mean that those servers are logging everything, but the possibility exists.
With armory, you are connecting to your own full node, therefore protecting your privacy as much as possible.