why does my computer sound like a jet engine when started up?

That box blowing out the air sounds like the power supply as you mentioned its not the fan, The reason why your computer is not turning on is because you have a faulty power supply, this has happened to me many times which is actually very annoying, replacing a power supply could cost a minimum of £30 which is not much,

P.S : Power supplys are very dangerous, don't go near it when your computers on haha :p

You said your not amazing with computers so im guessing you don't know how to replace a power supply, you can get a friend who is to do it for you if you purchase a new one.

This is also the key to your computer not starting up sometimes.

edit that: Don't go anywhere near the computer while its on haha, but don't get technical with a power supply when its on or off.

It sounds like a bad power supply. Unfortunately, some Dell power supplies are proprietary in design, as I suspect yours is. I don't think it will do you any good to take it to Best Buy. At best, they might charge you a lot of money to tell you that you'll have to call Dell for a replacement power supply (if that's what it is)

As long as you've had your computer, it should still be under warranty anyway. Call Dell.

It's probably the fan in the power supply. Usually the power supply has a built in fan. Try getting a can of air, and blow out all the dust. Maybe that will help. Otherwise, you might need to replace the power supply.

its the powersupply. dell power supplies are rubbish and if you havent upgraded it with the 280 you need to to have the power plugs that insert into the graphics card orelse its a wonder you havent blown your computer already. anyway buy a new power supply i have this one and highly recomend it:

http://www.amazon.co.uk/Corsair-Power-Supply-Versi...

opening up your power supply probably didnt help but i wouldnt use your computer till you plug a new one in as good chance tis gonna damage your system.

also your graphics card should make a loud noise when starting up as it tests the tempreture and fan speed on start up.

That small metal box is your power supply and it has a fan in it too.

Sound like problem coming from your power box, where your power plug attach to your computer case, btw it has a fan inside! not a costly, they can fix and change it in matter of seconds....

When you add data to hard disk it puts the data on any blank parts of HD, so when booting up programs that are defragmented and are all over HD it will make a lot of noise by skipping through the disks. Try defragmenting the HD once every two weeks or so...