Why is my computer suddenly so slow?

I was having the same problem (3 year old laptop) and I also have the most up to date virus protection, etc. My problem ended up being a cooling problem. Over three years my fan had stopped working efficiently and build-up within the internal components was causing the GPU and CPU to warm up more quickly. If you can open the case, clean the internal components and check to see if you fan is working properly, this should alleviate your problems (should run like it's brand-new if it's the same problem I had). Hope this helps and God bless