Dell has a built in diagnostics routine which is accessed by pressing F12 on boot up. Run this first to highlight any hardware issues.
I use Ccleaner to 'tidy up' computers. You can list programs that run at startup and disable any you don't need. Uninstall unwanted apps etc.
Run an antimalware program too such as Hitman Pro, Malwarebytes Antimalware or Adwcleaner (or all three!) They are all free to use, but you can buy them to unlock extra features.
What OS are you running? Windows 7? As Scotty said, you can use disk clean up as well.
