This will depend on the age of your computer and if you have any available RAM slots on your motherboard. If you have an older computer it may not be easy to find the right kind of RAM for your computer since it’s not all the same and will depend on the type of motherboard you have. You can take out a RAM chip/DIMM and see if it has a sticker on it saying something like DDR3 PC2800 to give you an idea of what kind of RAM you have.
If all your RAM slots are full then the only way you will be able to upgrade will be by taking out some of the smaller RAM DIMMs and replacing them with larger capacity ones. RAM is pretty cheap and easy to replace so it’s a wise investment if your computer does not have enough.
For Windows Vista and up you should have a minimum of 3 GB with 6 to 8 GB being the ideal amount.
When your hard drive is continuously spinning it could be caused by a couple of things. The first and easiest thing to do is to run a disk defragmenter on the drive (not applicable for SSD drives). When a hard drive becomes fragmented it has to search the disk in several different places that are spread out to get the information rather than in just one contiguous section. You should check it about once a month to see if it needs to be run.
Another reason a hard drive will be constantly working is because you do not have enough RAM in your system and the computer has to go to the hard drive for memory to make up for it.
I would also check your startup items to make sure you don't have any unnecessary programs starting with your computer using up your resources and running in the background.