This link has pictures of your PC. Next to the picture of the tower there are three thumbnails. Click on the second one to see a picture of the inside.
There is either a couple of screws on the back of the tower that secure the side panel or a sliding button with a small picture of a padlock next to it, remove the screws or slide the button to unlock and remove the side panel, sliding it towards the back (with the unit unpluged). Touch the metal of the case with you hands before you touch anything inside to get rid of any static electricity.
You can see the fan in the middle, check to make sure that is running when you switch it on and that it is clean and the heat sink behind it is clean. If it looks dusty, blow it out with compressed air.
Just to the right of the fan there are two vertical slots, these are your memory modules. There are two clips on each end of the slots, if you push the clips outwards they will release the module which you can then remove. Run the PC with just one module installed and see how it behaves, if no change, put the module back, push the clips back in place and then repeat with the other module.
If there is no change with either module removed you will need to test your hard drive.
On the left of the picture you can see yellow, red and black wires, these are connected to the hard drive. Look to see what make and model it is and put it in your next post, you may have to remove it to do this.
If this is all too tricky for you we can take a different approach.