Hey, just thoguht i would add my opinion. I usually leave my computer on all night all day at school all the time!
I see no affects of it slowing up due to the long times up. However, when i do restart for some purpose, or just because I feel it could use some cooling down. It seems to run better the next time i open it, for a little while. Maybe that is just superstition. But that is my 2 cents worth
Striderider