There are a number of possible reasons why your computer is running slower. It could be that your computer runs too many programs automatically at start-up, some hardware-related and others directly related to operating system contains hard disk issues causing low memory problems or is infected with a virus or spyware.
Viruses and Spyware:-
Viruses, adware and spyware can significantly slowdown a computer. Almost, malware will launch without you knowing it, the moment you turn on your system. Virus or Malware can also cause file corruption that may cause your computer to slow down. If you are noticing strange behavior accompanying the slowdown, consider installing, updating and running anti-virus, anti-spyware and anti-malware programs regularly.
Too Many Programs at Startup:-
Too many programs running at start-up can take up valuable memory space, causing your system to take a long time to boot up.
Low memory can be caused by hard disk issues. Fragmented data and unnecessary files on your hard drive's disk can slow down your computer's performance.
Another possible cause is the amount of RAM available in your computer. RAM is required to open, run and edit programs, and without enough memory, you will experience general slowdown, freezes and crashes. A lack of RAM may also prevent you from opening or using resource intensive software.
Various hardware problems could cause your system to run slowly. If it frequently runs hot, excess heat may be damaging the computer or otherwise impeding optimum performance.
High CPU Usage:-
If you are using up a lot of your system's CPU resources, your computer may run more slowly under the process load. When you go to open up more programs or pages, the system will react slowly because it has reached its maximum threshold for CPU usage.