In such cases, let us take both the sides.
1. Let us say the female is rightly complaining.
In this case, if we go to the concerned male staff and ask if he was doing anything like that, no one will be accepting the case. This is to be understood. The fear of getting proved guilty and facing the music will not allow the person to agree, and in addition to this, his image, his job etc are also at risk.
What is the solution then?
You will have to keep a close eye on both the people - the female who complained and the male against whom the complain is given.
Check what is done but don't make it evident to either of them that they're under observation.
If your office premises have CCTV camera installed, you can check the recordings and can use them as proofs as well.
You can ask the IT guy to check if data is being transferred from the female's PC to the male's PC and where are those stored.
Have a close watch but act as if you are not concerned in front of them.
In such cases, the normal practice is to first prove guilty and then take action. Unless you have proofs against the person, you can't give him warning letters.
Would appreciate if seniors can endorse their views on this.