What types of files are on there? I know if you have a lot to back up, this computer going slow can make it almost impossible to look through all of your files, depending on how slow you really are going.
Defragmenting, using crap cleaner, making sure to get rid of what you don't need is very important
But also, maybe too many programs are running at once. It is possible that too many applications are running at start-up, and a good way to find out is to see how much resources they are eating up. The Task Manager (Start-> Run: taskmgr.exe) will let you see what applications are running, how much memory they're eating up, how demanding they are vis-a-vis the CPU. You should also check the "Performance" tab to get a visual on how much resources you're using.
If you see any processes running with fishy names, check them
here by typing the name of the process (ex: explorer.exe) in the search bar on the right. The site will tell you what the process does, whether it's dangerous/necessary/useless.