Whatever you do, it sounds like you need to take stock of what is on your machine.
You may be able to remove much of the stuff you are seeing in Add/Remove programs, be sure to google first so you have an idea of what it is you are removing. Look at the folders in your Program Files, any you don't recognise dive into and have a scout around, look at any readmes.
Look at what is being started when you boot - one way to do this is Start/Run/msconfig and look in the Startup tab. You may be surprised at what you may be running without even being aware of it.
Try to familiarise yourself with the processes running on your machine - look at the task manager - right click on task bar, or Start/Run/Taskmgr - in particular the Processes tab. A lot of processes you won't recognise, but hopefully some you will. By clicking on the column headings, e.g. CPU, Mem Usage, you will be able to toggle lists in asc/desc order of which processes are using your system resources.
Finally, if you manage to get rid of significant amounts, defragment your disk afterwards. Also remember to clear your browser cache from time to time, e.g. weekly.