I post these suggestions assuming people run Windows machines, but some tasks work just as well for all computers. Try them adaptively.
I suggest doing an entire system backup before doing this laundry list of tasks. It's all relatively safe, but when it comes to your important files, redundancy can be your best friend if things go wrong.
You can do these in the order I post them in
1. Scan frequently for spyware and viruses. Unchecked, these can really bog down your system resources and compromise your personal information. Use Microsoft Security Essentials (free).
http://windows.microsoft.com/en-US/windows/products/security-essentials2. Disable/uninstall unneeded add-ons and tool bars from your browser. Dump your browser cache and history for faster internet surfing. Delete cookies. Use Glary Utilities (free). Glary Utilities is a multifunction program designed for improving your system performance, which I will refer to for a few of these tasks.
http://download.cnet.com/3001-2094_4-10508531.html?spi=6b6042efb5c25b6ded723ca3e9183f123. Turn fancy windows effects off. They take up processor power. Run basic or classic graphics themes.
4. Turn off redundant system services and disable unneeded start-up programs. Type MSCONFIG in your start menu search/run bar to do this. Glary Utilities also has a function for this.
5. Uninstall unneeded or redundant programs, and delete their remnants. Use Windows Uninstaller. Glary Utilities also has a function for this.
6. Clean junk off your hard drive, everywhere you can think of. Store personal and downloaded files on a drive other than your system drive. Use Windows Disk Cleanup, and/or Glary Utilities.
7. Clean and defragment your Windows
registry. Use Glary Utilities for this. Windows has no built-in program for accomplishing this, but if you've never done it before and you've been running the same operating system for a long time, this single task can work WONDERS.
8. Keep your hardware drivers up-to-date, along with your operating system updates.
9. Defragment your hard drive. *Later versions of windows do this automatically. **NOT NEEDED OR RECOMMENDED FOR SOLID STATE HARD DRIVES.
10. Spend some money and upgrade your hardware. If you use a solid state hard drive for your operating system, you will notice a significant decrease in load time. If you upgrade your memory, motherboard and/or CPU you will notice a significant improvement in system speed. Getting a graphics card will improve visual and gaming performance.
11. If your hardware supports it, you can try overclocking it--but be careful and do your homework before you mess with this. Consult your hardware manuals for this.
12. If your computer still runs way slower than you think it should you can always back up your files, wipe out your system drive, and do a fresh Windows install. Keep in mind this will wipe everything out. You will need to reinstall all your programs and back up your important files onto a different drive, but having a faster computer may be worth it.
I suspect your computer experience will improve by leaps and bounds if you do these tasks right.