One common piece of advice is to run the VM off a secondary physical HDD, especially resource-intensive OSs like Windows 8. There are multiple reasons why, which you could easily Google. The most important being the lifespan of your drive and the prevention of data loss from a PHYSICAL HDD crash.
It is NOT RECOMMENDED to run a VM on your primary PHYSICALL system drive.
I noticed a performance improvement after doing so even with a poor i5, 8G, x64-based notebook.
Also, if you can afford, get yourself an SSD drive, it will greatly improve your performance.
For the record, I run my VM on the above specs, with an external, ordinary IDE drive. It suits me very well though, I would much prefer better configuration. On my next upgrade from my 2009 hardware, I will be kicking it. My present for me this christmas