T 3400 desktop running so slow, even drying paint is faster

Hi everyone, I have a dell T3400 desktop which has 8gb of ram and have had it for about 3 years now. Turned it on earlier and it was really slow. Far slower than my original 286, which is like a greyhound compared to this. Yesterday, it was working fine. It's taking about 10 mins just to load up. Then I click on something and takes so long, that I gave up the will to live! Has anyone got any ideas? I'm running windows 10 and never had any problems till today