RE: Alienware M14x R2 Hard drive failure - UK
See above for the instructions in deassembly. £120 is reasonable for the cost of the drive and the labour costs however a repair shop will likely install a 2.5" HDD opposed to a 2.5" SSD.
If you want your system to perform substantially better you should get a SSD for example the Crucial MX200:
You could also get a hybrid drive (mid performance):
The mechanical drive is the lowest performance:
For Windows 8.1 installation see here: