Windows is so slow and HDD Led is on always.

Hello.

My Windows 7 SP1 is so slow and even I can't open my computer :(. My system is Core i7 with 8GB Ram. I checked processes and Performance via task manager is everything seems good. My Antivirus is updated too and my system is not infected. Why system is so slow? When I see my computer case then the HDD led is always on.

Any idea?

Thank you.