How can I get the computer I custom built and paid for in full that my ex-girlfriend is using at her grandma''s house.

In Virginia, when you want to get a court to order that someone return something to you, you can file a Detinue action in the local general district court.  Detinue is a fancy way of saying you want the court to order the defendant to either: 1 give you the thing in question or 2 give you the fair market value of that thing.  Once you go to court and fill out the paperwork, and have it served on the defendant, the court will conduct a trial.  At trial, you will need evidence that you purchased all the components, and will need to testify as to the value of the completed computer, and  that you did not give the computer to your ex girlfriend.  Your exgirlfriend can then put on evidence to defeat your claim - such as she may claim she bought the parts, or she may claim you sold her the completed computer, or that you gave it to her as a gift.  The judge will then rule on whether he believes you proved your case.