If you're looking to replace your desktop hard drive it might be a good opportunity to upgrade to something like an SSD (Solid State Disk). Kingston provide what they call an "SSD upgrade kit". This is good because it includes the replacement hard drive, 3.5" mounting brackets, an external USB case, and most important some disk cloning software.
All you do is:
1. Pop the new SSD into the USB case.
2. Plug the into a USB connector on your desktop PC (PC should be powered off)
3. Turn on the PC and put the cloning software CD in the CD drive
4. When booted from the CD drive follow the instructions to clone your existing system
5. Once finished shutdown
6. Replace your old driver with the new one
7. Finished - you have a new SSD drive in place of your old disk drive
I just did this with my laptop and it works great. SSD is so much faster than traditional hard drives (but is more expensive).
Alternatively take a look at the Seagate Hybrid drives:
http://www.seagate.com/gb/en/intern...-hard-drives/laptop-solid-state-hybrid-drive/
They are a good compromise between performance, capacity, and price. They combine a small SSD with a larger hard disk on one package. Giving many of the performance benefits of SSD without the much higher cost. And capacity is greater at up to 1TB on a 2.5" drive.