Your hard drive still has all the "old" drivers from your previous hardware such as chipset drivers, sound drivers, and video drivers, etc.
Of course, this is an assumption on my part, that the hard drive you swapped from your old system was the main "C" drive. Am I right?
You installed Windows 7 Professional. You're using a Gigabyte motherboard also, good choice by the way! Go and download all the latest version drivers for your sound chip, chipset, video card and such. Store them on a thumb drive, CD-RW or wherever.
Do a clean install of Windows 7 Pro, it will run so much better than it does now.
I am assuming your installing the 64 bit version.
The following are the latest Realtek drivers you'll need.
For your Realtek soundchip drivers go to:
Realtek Sound Drivers
For your Realtek LAN 8111C drivers go to:
Realtek LAN Driver
Go to the
Gigabyte page for your motherboard model for the chipset and SATA drivers
Use the link on the previous post for the ATI Radeon drivers.