Tech companies in Seattle are clustered in a few different neighborhoods, but most of them are walkable from their respective neighborhoods. That said, tech companies are slowly migrating toward the greater downtown area due to the gravitational pull of techies that want to live there. And everything in the greater downtown area is walkable if you don't mind walking (and people in Seattle don't). It isn't a small area.

Microsoft specifically, though, is still mostly on the "east side" (the metro area has a 88 square kilometer lake in the middle, Seattle proper is between the "west side" of the lake and the bay). Microsoft has offices in the major urban neighborhoods but still have a huge campus in Redmond (an east side suburb).