How big is your company? If your company has an HR department- by all means- go to them. Prior to going to them- I'd write up what specifically the other person has done & see if there any relevant e-mails.

If your company is not large enough to have a HR department, then you're going to weigh your options. What type of reputation does the other employee have? Does the other employee have the same job as you?

Completely understand what you're going through- it is terrible. But the important thing is not to lash out & figure out a good strategy to get yourself out of this situation.