Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things, and are always capitalized.
Since "computer lab" refers to a general type of room, it's a common noun.
However, if you're referring to a specific computer lab with a name, then that name would be a proper noun. For example, "The Smithsonian Computer Lab" would be a proper noun because "Smithsonian Computer Lab" is the specific name of that lab.