Computer is extremely slow.

I know this question has been asked many many times already. But for my computer, when I start it up, it is stuck on windows logo for over one day. If I am lucky it opens one day and tries to repair itself but shuts down automatically. If I am extremely lucky, it opens up, but each click from my mouse takes like 5 minutes to respond. for example, when i try to open documents, it takes like 5 minutes and when I try to open a file it takes like 5 minutes to open.
This computer is like over 4 years old. (not surprising because I have a smooth working laptop which is 7 years old). My computer is Intel motherboard, 4GB RAM, Dell monitor.
How do I fix this?
you can email me personally on 'removed by moderator@yahoo.com'.