There's a number of possible issues:
- overheating
- malware
- overloaded system
- bad drivers configurations
- old computer
for starters you want to make sure you don't have any malware
Free anti-virus:
-
avast
-
AVG
-
Microsoft Security Essentials
Free anti-spyware/anti-malware:
-
malware bytes
-
spybot search & destroy
-
super anti-spyware
after that I'd clean out any dust you can get to with compressed air and check the temperatures after that with Speed Fan or HW Monitor
if it's still not fixed, I'd check what programs are running in background and shut anything down that is not essential to your usage
lastly, if this is a 6+ year old system, face it, it's old. You can't expect it to perform the same speed as you bought it some years ago.