Hi there, Welcome to NaturePlus!
We called these "Daddy Longlegs" when we were kids. http://en.wikipedia.org/wiki/Pholcidae.
There was an episode of Mythbusters that showed these spiders...
It isn't actually a spider. It's a harvestman which is an arachnid but not a true spider. They have only part to the body, not two like spiders. It's not a member of the Pholcidae which are spiders.
I don't know the species sorry but there are many common ones.
So i take it its not dangerous.
Should i leave it or just put it outside?
They look pretty big and mean to be honest and im seeing a lot of them appear in my house.
Is there anything you can do to deter insects/spiders/arachnids from coming in?
Definitely not dangerous. I've never heard of harvestmen even coming under suspicion. Most are opportunistic feeders and include plant material in their diet, though a few are active hunters.
It shouldn't be in your house so just put it outside. I don't know why they come in your house, maybe you just have a lot of suitable habitat near to your doors and windows. I think most of them like it damp and dark.