Uninstall any obsolete application.
Uninstall any third party toolbars. Most are uterly useless, several pose serious security and privacy risks.
Clear up the junk files. Get and install CCleaner. It's free, thourough and safe.
File clutter can realy bog you down.
This application can also scan your computer for various faults and errors of the registry and correct them. To be realy safe, save a backup to some external support.
To be run about monthly.
File fragmentation KILL performance. Get a good defragmenter as the one included with Windows is seriously lacking. I suggest Defraggler from the maker of CCleaner. A very good and free defragmenter.
If you are ready to pay something, there are several premium defragmenters available proposing several advanced options, like smart, "set and forget", scheduling that ensure that your drive(s) stay optimised.
Defragmentation must be performed at least weekly.
Get and use some decent antivirus. Many, like AVG, Avira and Avast, offer a very good free version. There are several others. Take a look at download.com. Normaly, a weekly scan is good enough.
You probably have to many tasks running in the background.
Check your startup items. Disable all those you don't realy need or use. ALL quickstart/quickstarter and similar are uterly useless.
Check the notification area. You should have the sound, network, antivirus and less that 10 other icons there.
Check them all. Any you don't realy need or use MUST be set to NOT start automaticaly.
Electro, October 2013