Hi,

As already suggested, adding a drive will not neccesarily speed up your computer. However, you should check to see which drive is fastest if any. If there is a difference, I would make the fastest one the "C" drive with the main operating system and applications on it. Use the secondary drive for data and stuff that you do not use very often or is intrinsically slow, like office applications.

Make sure that you defragment regularly.

As mentioned, a RAID setup is faster (almost2x) but only if you use RAID0 (striping). In this configuration you are using both drives simultaneously. They should be of the same type size and speed, and you need a motherboard that supports RAID. I suspect that you do not have this, and your drives are probably old, so RAID0 is not recommended, as if you lose one HDD you are history

You don't tell us much about your setup, but I would look to increasing RAM as the easiest way of speeding things up.

Cheers