Dells has a variable speed fan that run at max (loud) when the temp is way too high (ie u r blocking the fan flow or it is massivly dusty) or when the CPU is running at above ~50% for a sustained period, or when the HD is getting hit over and over. (not enuf ram for all the proggies open... or a massivly FRAGMENTED HDD/swap file can be to blame)

Other replier is correct to suspect spyware/an active antivirus scanner, P2P or some other resource intensive process running in the background. Defragment, and check out the status of ur swap file. Diskeeper is better than the Windows defragger.

Good Luck.