(Redirected) DELL INSPIRON 15R SE 7520 i7 RUNNING VERY SLOW

Hello. Sorry for reopening this topic with the same name. I couldn't comment on the other post.
So basically i have the same issue as this thread:
http://en.community.dell.com/dell-groups/new-to-community/f/3511/t/19484572
I tried the solution from there, i change the boot to UEFI etc. But then the windows couldn't load. It said ina  black box Checking media [Fail] and than it restarted it self and same in an infinite loop. 

Please help.
Thanks.