Dell Inspiron 15z running extremely slow!!

4 days ago my laptop (Ultrabook) was running perfectly fine. Then all of a sudden, while I was working in excel, it started to run extremely slow, so slow it was unusable. So I contacted Dell who sent out a new Windows 8 disk.

I received the disk yesterday, and booted from the disk in UEFI secure mode. I was then asked to provide the drivers for my hard drive. So I downloaded them from the intel website on my other laptop and installed them using a USB stick.

I was then instructed to choose a disk drive to install windows to, so I deleted ALL partitions and created a new one for the OS. Then windows created it's own partitions (system, recovery, etc).

Then I left the Windows installation to complete, this was taking a very long time so I had to leave it over night. It took over 5 hours to finally install. 

After it has installed, I tried to boot into windows, which was still running extremely slow, so after 2 hours I gave up and turned it off.

I have run a diagnostics, which shows no problems with hardware (although the thorough memory test is still running). I do have next day in home rapair from Dell, that should run until 2016, but for some reason my warranty displays as expired, so when I rang them they wouldn't do anything (which I will be complaining about on monday!!!)

Please could someone shine some light on my problem, I am usually great on computers and have always been able to diagnose and fix problems myself, but I have no experience with UEFI or SSD drives used for cache, so I'm stuck on this one.

Any help would be highly appreciated.

Thanks in advanced.