I have a dell GX280 desktop and the noise from the fan is terrible and when it starts it cuts the computer off and when i restart the computer it says hit F1 to start the fan is at risk how do i correct this fit a new fan and how do i know which fan any ideas
As lotvic says, you need to remove the side of the compuiter to have a look at the problem. It could be something as simple as a cable has fallen /moved and is 'snagging' the fan, causing this problem.
Before you do this, unplug the computer from the mains( for your own safety).I would advise you to get an earth strap and earth yourself against a metal object before starting (reduce static).
You can also while you have it open, have a look at the heatsink (the silver box shape above the processor) as it may be full of dust.
Check the fan to see if it spins freely, if not you will need to put a small ((2 drips) of a fine oil (wd40 is OK)on it.