Do these steps and you should see an improvement:
Upon resetting your PC, before opening up any applications.
Open the Task Manager (CTRL+ALT+DEL) // (or type 'Tasmgr' in the windows search bar)
Go to the Process tab and monitor your CPU usage, is it constantly fluctuating between 10%-50% or higher?
Most likely you have too many applications running in the background, possibly even spam/adware and other stuff 'clogging up' your PC and 'hogging' your CPU.
First step is type 'Msconfig' in the windows search bar & press ENTER, a little window witll appear with several tabs.
Check out Startup/Boot/Services.
Under either Startup or Boot, disable any & all applications you do not want to load when your PC starts up.
Secondly go to the Services TAB & make sure the 'Hide all Microsoft Services' box is checked.
Now you should see a list of 'Services' with a column for (Running) or (Stopped) - the serviced which are 'Running' are what's taking up your CPU usage.
Disable things like (Dropbox Update Services) or any other 'Service' you do not want to run/are not likely to use on a regular basis.
Reset PC & you should see the results INSTANTLY.
Bear in mind doing this, you will have to tick back the box for the application & reset your PC, if there is a particular service/application you want to load but have disabled (or else all you'll get is an error messsage like "application error|could not load application".
Finally, install an Antivirus like AVG FREE version.
The reason why I recommend (highly) AVG against Norton or McAfee is because AVG is a lot lighter than other Anti-Virus software.
The next step is to temporarily install (keep it if you want, but disable it in the Services as this running in the background takes a lot of CPU resources)
*MALWAREBYTES
Do 1 full system scan and remove all 'infected files', then delete or disable the service VIA MsConfig.
Once you've done all these steps come back & let us know how you got on.
Ah, one more point! How much RAM do you actually have?
I hope you have a minimum of 4-8Gb (8Gb minimum being ideal).
Worth checking you got the latest drivers, could be that CsGO latest updates or your RX460 latest driver updates conflict with the game/GPU?
Another point worthy to note is that browsers like Chrome or Firefox take up a lot of RAM/system resources.
Try going lite by considering Opera Neon, very simple/light browser, compared to most.
EDIT: obviously opening up the PC and checking for dust is a good idea, but if it's only bee 3 months, it shouldn't be that bad - still, a worthy point to mention.