My computer runs very slow. How can I make it run faster?

First thing:
Get rid of un-needed startup items. Many addons make your computer run slower and take much to long to boot.
Disable any "Quick starter" and "Preloaders". Mostly, they can only offer any benefit for about 20 minutes after they are loaded, and some will reload themselves every few minutes, greatly slowing you down in the process.
Some startup tasks are options of usefull applications that should be disabled, other can, or should, be simply uninstalled, like old, no longer used, accessories and applications.

Clean up the various temporary files. I regularly use CCleaner for that task. It can safely purge your drive(s) from most leftover temporary files. It's free.
A drive clogged with to much files become less performant.

Once you've done some cleanup: Defragment your non-SSD drive(s). You should NEVER defragment SSD drives: It uselessly shorten their lives and don't improve their performances.

The Windows default defragmenter is not realy performant. Take Defraggler. It's an excellent and free aletrnative. If you are willing to pay for it, take Diskeeper amd use it's "Set it and forget" feature.
File fragmentation have a HUGE impact on your performances.

Do a virus scan. Any virus slow down your computer as it steals your resources to do it's nefarious deeds.
There are various very good free andivirus available. An antivirus is NOT beter because you need to pay for it. I personaly recomend AVG or Avira. Both have very good free offerings that surpace some well known payd AVs.
The scan will be faster on a cleaned up and defragmented drive.

Do a spyware scan. As virus, spywares can slowdown your computer. They also steal personal informations and push targetted adds to you. They may also push some virus disguised as "good deals"...

In your browser, DISABLE any additional tool bars. If any toolbar have an uninstaller, uninstall it. Most, if not all, third party toolbars are useless, often duplicating functions of the basic toolbars and several menu items in a not so convenient maner.

LAST, and often NOT REALY needed:
Do a registry cleanup and defragmentation.
Be carefull here, as if done incorrectly, it can have adverse effects on your system.
CCleaner can do that task relatively safely.

Electro
August 2012

Varies on operating system, but you need to get rid of *.tmp files, defrag the registry and the hard drive

SD
July 2012