Your PC may be overloaded from too many processes running in the background, and without proper software to protect it, such as an Antivirus it may become infected with viruses, spyware , malware, etc.
Make sure your Antivirus is up to date, or if you don't have one you could use Avira Free Antivirus, it will keep you safe from viruses, adware, spyware, Trojans, rootkits, dialers and many other types of malware that may infect your computer.
Also, as we install and uninstall programs, over time our hard disk(s) and registry become fragmented making it harder for the PC to access information thus slowing you down.
You could try to defragment your hard disk by going to My Computer > right click on a partition, for example : Local disk (C:) > Properties > click on Tools tab > click Defragment Now... button , and make sure you defragment all the partitions you have.
There is a software I recommend that does this task and also has many other features that will optimise your system, making it run faster, called Advanced SystemCare. They have a free version too, so you won't have to pay for it.
