word choice - "If it was" or "if it were"?

Should I say "If I were [something]" or "If I was [something]?"

This came up because I am writing a comment above a function in some code. Here's the comment, basically:

// Shuts down everything
// Will stop the process if it was running
function Shutdown()

So should that say "if it was running" or "if it were running?"

Also, why?