The victim, 29-year-old Timothy Neuer, was shot at a home in the 300 block of Alexander Valley Road that he shared with his girlfriend, Amanda Miller, sheriff's Lt. Dennis O'Leary said.
Miller's brother, 29-year-old Jarrod Miller, had recently moved out of the house, O'Leary said.
Amanda Miller told sheriff's deputies her brother was at the house Tuesday night, O' Leary said.
"She said there was no argument. Everything seemed fine until he started shooting," O' Leary said.
Amanda Miller reported the shooting to the sheriff's office around 11:20 p.m., Lt. Steve Brown said.
She left the house and closed the front door after the shooting, and was outside on the property when deputies arrived, O' Leary said.
Deputies broke down the locked front door and found Neuer dead on the floor, Brown said. He had been shot several times.
Miller told deputies her brother had left with a handgun in a white Chevrolet Suburban, Brown said.
Windsor police officers spotted the Suburban traveling south on Old Redwood Highway north of Limerick Lane and arrested Miller, Brown said.
He told the officers he had thrown the gun out the car window just before he was pulled over, Brown said.
A California Highway Patrol officer found the small-caliber handgun in a vineyard about 500 yards north of where Miller was stopped, Brown said.
O'Leary said a "substantial" indoor marijuana grow was found in the house where the shooting happened. He said investigators don't believe the killing was related to the marijuana grow.
The sheriff's office is still investigating the motive for the shooting.