Hi if the air vents did need hoovering the inside probably needs cleaning as well.Especially the PSU and CPU fans.Check that both are running Ok.A Fine haired paint brush with a hoover with plastic nozzle is what I used.When nothing running press Ctrl.Alt and delete at the same time.Takes you in to TaskManager. processes.Under CPU system idle process should be showing About 97% all the rest the remaining 3% If anything else using a high% of the CPU let us know what.
what are the specifications of your machine. Is it a branded machine or a purpose built one. Is it a laptop or a desktop machine. What operating system are you using. What anti virus software do you have What is your ISP. What is running in task manager What is the total memory and the memory available in task manager.