Run a Reliable Spyware Cleaner
With the growth of media and software downloads, there's a good chance there a file on your machine is associated with spyware or ad-ware. Even if you're running an anti-virus system, you might be missing out on detection of files that can slow down your system and open up potential software vulnerabilities. Most spyware systems work by connecting your computer to an external data-mining source, allowing advertisers to serve you targeting ads without your knowledge and taking up valuable computing resources. In addition to running a system cleaner, consider deleting your browser's cookies and off-line files regularly.
Do an Audit on Your System's Start-Up Programs
Upon installation, many programs start-up with Windows by default. Even if they are passively running in the background, this can take up quite a bit of memory and system resources. You can review these programs in the Control Panel, and can turn off automated startup for those programs that you don't use often; this can help you reduce both start-up and load times. In Vista, you can find these settings within Windows Defender in the 'Software Explorer' category in Tools. From there, you can then disable automated startups for programs that you don't use.
Consider Defragmenting Your Hard Drive

Over time, new software installations can be fragmented across various points on your hard drive. As a result, it can take longer for your processor to access files – to correct this problem, you can set your computer to defragment its hard drive on a regular basis. You can do so within the Windows Task Scheduler to set a weekly or monthly run time. After installing a particularly large program, such as Photoshop, you may want to also run the process manually.
Keep Your System Drivers Updated
Many slowdowns in Windows relate directly to out-of-date or incompatible software and hardware drivers. In particular, check to see that your graphics card has an updated driver that is optimized for your OS, especially if you're using Vista. By checking for new hardware drivers, you can greatly improve run-time speed and reduce incompatibilities that may slow down your system.

Run Only Those Features of Windows You Need
Although Windows comes with a number of features turned on by default, you may not need to run all of these various programs. To check to see which features are turned on, check in the Control Panel. In the 'Uninstall a Program' and click on the 'Turn Windows Features On or Off' tab on the left sidebar. Here, you will be able to select which optional features are implemented on your OS, and can improve the speed of your operating system by turning off features that you don't need.