Answer:
Small, containing light, You'll need it in the dark, It will provide that spark, and shine into the night. Will light up any pyre, 'Cause it can help make a fire.
I occur twice in eternity. And I'm always within sight.
The line a runner strives to reach
It covers you, you lose it every day and you'd die without it.
I make billions of dollars selling Windows. Who am I?
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.