The best and most reliable way to speed up your computer is to reinstall the operating system.  Yes, I know it is a hassle and an extreme option but it’s the best option.  You can try to manually cleanup your computer and you can make improvements but this is time consuming and no gaurantee you will fix the biggest problems.  Reinstalling the OS requires that you back everything up which is a good thing to get in the habit of.  Your computer could crash one day and you should be prepared for this anyway.

Reinstalling the OS takes a few hours but it runs pretty much unattended.  The most work will be preparing for the reinstallation.

But if reinstalling the OS  isn’t an option, here are the things you can do to improve performance:

1. Remove spyware using Windows Defender or AdAware
2. Remove viruses using Microsoft’s free scanning service or installing Microsoft’s free Security essentials  software.
3. If you are low on disk space, free up disk space using Disk Cleanup or WinDirStat
4. Defrag your disk
5. Detect and repair disk errors
6. Make sure you have enough RAM
7. Uninstall unused program
8. Remove unneeded services
9. Remove programs that auto-start
10. If your browser is causing problems use a different one.  You may inadvertently have installed many plugins for Internet Explorer which is slowing it down or causing problems.  Firefox and Google Chrome are excellent alternatives.

For more ideas checkout this articles.