Understand what a complaint is -- a good faith allegation that a company policy and/or the law has been broken. Examples include discrimination and harassment, wage and hour issues, fraud and theft, and health/safety/environmental issues. We say "allegation" because employees suspect there's a violation but may not have all of the facts or may not know how to apply the policy or law.
HR has a LEGAL DUTY to investigate such compliance matters on behalf of the company. Therefore, refusing to participate in an HR employment investigation may interfere with the company's obligations to uphold its legal responsibilities. Cooperation and truthfulness are usually expected and required of employees during investigations. You're starting to get into the territory of insubordination if you refuse to participate or are dishonest during the investigation.
For similar reasons, once you raise a complaint, you can't simply say "never mind" or "I take that back." HR has a duty to investigate and see it through. The desire to "unring the bell" is more frequent than you'd think.