When compound modifiers follow what they're modifying, they don't require a hyphen.
Are you then saying that when they precede the word being modified, they do require a hyphen? This is getting too complicated! Maybe we should submit it as a 5th Grade grammar question to Fox Network.
It would be virus-free computing but computing virus free. Compound modifiers are hyphenated when they precede what they modify(unless they're really familiar terms like high school) and unless it would read too ambiguously unhyphenated if they come after what they modify. An easier example might be "the well-read book" vs. "the book well read."