You may have a lot of programs running in the background on your PC. These programs eat up your system memory. First, have a look at the performance of your memory by doing this:
I like to perfrom this process right after a hard boot so I can determine what state my system is in when I first turn it on...not after I've been using it all day. Press "Alt" + "Ctrl" + "Delete" keys on your keyboard at the same time. Click the performance tab in the task manager. Have a look at your memory and see what its activity is like. Are you low on Memory? If you are low on memory and have performed these steps after a hard boot, you have a lot of these little programs running in the background. What are they and how do you take care of them?
Click "Start", "Run", type "MSCONFIG" (Thats the Microsoft Configurator), and press "Enter". When the MSCONFIG program opens, click on the startup tab. The programs in this list are the programs that run when your computer boots. The names of these programs may seem like they're written in a foreign language, but a look at the programs path may help you determine what program it is. Uncheck all of the programs you know you can safely remove. If you are unsure about a program, either google it or leave it checked. When you have removed everything you need to, click "OK" and then reboot the machine. When you get back to your desktop, a message will appear letting you know that you have altered the startup options for your PC. Check the "Don't show me this again" message and then click "OK". Now press "Alt" + "Ctrl" + "Delete" keys on your keyboard at the same time again to bring up the task manager. Have a look at your memory and free system resources now. Any different?
Hope this helps.