If your computer is running slowly, press Ctrl-Alt-Delete and click on the 'Performance' tab. While I'm typing this, mine is currently showing CPU usage of well under 10%. (Typically around 4%). If the figure you see is much higher you need to investigate further. Click on the 'Processes' tab and look down the 'CPU' column. You should expect 'System Idle Process' to be showing just under 100 but (unless you're running some very heavy duty applications) everything else should be '00' or a very low number. If you identify something which is hogging your CPU you can either use 'End Process' to close it (if you're sure that it's not something which is essential to the well-being of your computer) or post again, telling us what it is, so that we can advise.
Here's one last suggestion for now:
Go to Start > Run. Type 'msconfig' (without the quote marks) into the box and click 'OK'. Click the 'Startup' tab and examine the list of programs which are running automatically every time you start your computer. Click to remove any unnecessary ticks. Click 'Apply' and 'OK'. Restart your computer. You'll see a message telling you that you're using 'Selective Startup'. Put a tick alongside 'Do not show this message again'.
Chris