There's several tools and things I do on multiple computers a week to help speed them up. First things first, use the Disk Cleanup utility to remove temporary files from your
computer. This utility is in the All Programs -> Accessories -> System Tools folder in your Start menu.
Next, turn off any unnecessary startup items using MSCONFIG. Now, this is something you have to be careful with, and if you don't know how to use it and what specifically needs to be turned off or left on, then I'd suggest not messing with it, but it's what makes the biggest difference in system performance I find. Use the startup tab to uncheck the items that don't need to start up automatically in
Windows, such as MSN Messenger, Quicktime, Adobe Reader, Microsoft Office, the list goes on and on. The less you have loading at start up, the faster the entire system will perform.
Next, you can run a scan using the program Malwarebytes. It's free to download from
http://www.malwarebytes.org and is pretty straight forward. This will search for and remove malware, beyond just viruses, and get rid of a lot of things that are hiding in the background using up resources, using up network bandwidth, and possibly even monitoring your usage and sending it off to somewhere else. Let it scan and then remove all of the threats that it detects.
Now, from there it really comes down to a more customized system. What kind of operating system are you running? What
computer hardware? What anti-virus software do you use? Details like this can determine what else can be done to help fine-tune performance.