How do I temporarily disable applications from starting up at login?
Hold the shift key as the user log in process starts.

You need to hop on the shift key quickly if you enter a password right after pressing return, get on the shift.

If your account logs in automatically, look to get on the shift when you see the gray screen with the dark grey apple logo. Next comes the blue screen and then the graphical user log in portion. If you hold the shift too early (before the Apple logo) you will have a safe boot(not what you want) 

A safe boot has much more temporarily disabled than just the user log in items.

See the Apple knowledge base article for more information about items that open automatically at start up and how the shift key trick works.