Sometimes women seem happy to talk to me.

Other times they seem startled, confused and a caught off guard... how can I stop this? Am I too aggressive?

For example, this gorgeous girl was entering the lobby of my work today. I was on the way out when I saw her. I immediately altered paths and walked right up to here and said "How are you today" and walked with her to the elevator. We talked for maybe 30 seconds... I managed to figure out where she works and on what floor, get a handshake in an introduce myself, then the elevator arrived... maybe I should have tried to stop her from getting on it? In any event, she seemed pretty startled throughout the interaction to be sure.

I couldn't tell if she was annoyed or not... it all happened too fast.

When I see her again, at least I have name with which to strike up a conversation.

How can I improve this and have immediate comfort when approach women?

You are over thinking too much.

Being caught off guard has nothing to do with you. She was casually walking to work thinking about how she looks and how her diet is going and what her friends are saying about her behind her back, and you confidently walk over and say hello.

The correct amount of aggressive is attractive.