Pre-fab PC or some homebrew you had someone build for you?
Open up the side of the case and check then.
If there's a whole lot of dust you're better off cleaning out the inside some. If it's really coming off your HD (the sound that is) then it could be going bad.
Run chkdisk (that will take a while, do it over night or something) and that'll let you know what shape the drive is in.
Other than that it's the fans that would be causing the most noise.
Sounds like a HD problem to me though, good luck.