Berbatov took just two minutes to set United on their way to a 5-0 win and extended the unbeaten record of Sir Alex Ferguson's men in the league this season to 22 matches.
The Bulgarian headed home after John O'Shea flicked on a Giggs corner and added his second just past the half-hour mark when he pounced on Roger Johnson's error.
Giggs slid home a simple chance in first-half injury time and the misery continued for Birmingham after the break as Berbatov converted his hat-trick after 53 minutes and Nani's low effort on 76 completed the rout.
Source: PA