Hi Sergio,
Couple of tests to do:
Bring up the task manager, click on the Process tab, wait until the System Idle Process is 98-99%. Then switch to the Performance tab. Load a program or read a large file, and see if the CPU is nearing 100% utilization. If so, switch back to the Process Tab and see what process in hogging all the CPU time.
Other thing to do is to download and install HD Tune (or Pro). Choose this HDD. Click on the Health Tab to see if there are any SMART issure identified. Then click on the Benchmark Tab and let it complete. Report the Max Read rate, avg rate, min rate. It should drop from max to about half over the disk surface. Report the values or you can copy that screenshot, and upload it for us to see at your post back.
Finally click on the Error Tab, (could take a couple hours for a full scan) and see if there are any errors from the disk surface.
That may give you the info you need to find out why your HDD is running so slowly.