hard drive plays no roll in the speed of the computer. Your Processor takes on all the work load. If your system is bottlenecked by the Processor everything else will bog down once the system gets cluttered with files.
To help with this is get a spare external hard drive and keep all pictures, videos, music on it. Leave the main C drive that contains the windows files free of clutter to better help the CPU with processing HD requests. Dont get me wrong if you were to buy a SSD your system will load files faster, just wont make the system as a whole faster
Upgrading to another OS by using the upgrade function will not help anything, as this keeps all your files intact from the previous version of windows. Its best when going from say vista to windows7 is to do a complete fresh install. This will WIPE the hard drive 100% clean of all your files. So you can start fresh.