mass effect 2 archangel gun wont fire at mercs

I have the PC version of Mass Effect 2. So far I have experienced two problems: difficult to keep target for the 'use' option (in my case the spacebar), and a glitch in the Archangel mission.

1. I could be looking right at the item I want to use, but the blue box indicating that I can press the spacebar flickers. It never stays still, even when I haven't moved the mouse. I thought it was just my keyboard, so I switched the hotkey for 'use'. That gave me no results. I changed it back to the spacebar and it still makes it difficult to open or to grab an item.

2. I arrived on the planet Omega to recruit the Archangel. I do all the needed work and arrive at the bridge, but I can't start firing at the mercs to help the Archangel. I aim and I press the button to shoot, but it does nothing but let me see the health of the person I'm aiming at.

I really love the Mass Effect games, but these bugs make it difficult, if not impossible to continue the game.

Thank you for your time.