and there must be too many programs start up automatically when you start your computer.Run a registry optimizer or cleaner to stop those unnecessary start-up programs.

Have you ever clean up your computer since you bought it? If the answer is never, I am sure that there are lots of junk files on it. Junk files is also one of reasons that will cause computer slowing down. You’d better delete them regularly.

If all don’t work, add RAM or do a fresh reinstall of windows. If you want to improve your hardware, you can add some RAM but not buy a new hard driver.
Before you do a reinstall, please remember to back up.However, you should know that frequent reinstall is harm to hard disk.

Part 1: For the software cause
1. Startup overload
2. Improperly BIOS setting
3. Bloated windows registry

Part 2: For the hardware cause
1. Insufficient memory
2. Badly Fragmented Hard Drive
3. For the CPU problem

Part 3: For the Spyware & Viruses cause

Go to Multi-method to Speed Up PC Performance:
http://www.system-tools-software.com/knowledge/multi-method-to-speed-up-pc-performance.html