Aren't they frustrating?

There's a couple of people I come across at work who make me tense up as soon as they walk into the room 'cos I know full well they're going to make mundane questions, ask me really annoying questions and request that I help them with tasks that, despite being straightforward in theory, they manage to turn into the most complicated thing on the planet.

However, because they're attempting to be nice, etiquette means you can only be nice back.

After they leave I'll
a) be in a tense, bad mood
b) feel guilty because they've actually been trying to be nice and it's a bit unreasonable for me to be as pissed off as I am with them.

How do other people hope with this kind of person?