This all started about a year ago so I think I can rule out a failing drive...

Dell Dimension 8100
1.4 P4
128MB RDRAM
Nvidia GeForce2 32MB
Seagate ST320413A 5400rpm 20Gig HDD
(Not the best specs but was perfectly adequate b4 it started slowing)

I had no problems until my PC started gradually slowing, couldnt figure out the problem so I formatted(not a problem as I do it quite regularly) and all seemed fine again...for a couple of weeks, it just gets slower and slower until I give up and format however recently its slow as soon as I've formatted and getting slower.

The latest benchmark I ran gave a performance equal to an ATA U33 5400rpm 10Gig drive, I dont know whether thats particularly slow(I was expecting it to be closer to a floppy as thats how slow it is).

From the point where the desktop appears on startup until disk activity stops takes about 2 minutes. The disk itself is a lot louder than it used to be(drive is advertised as being almost inaudible! sounds like a train).

Everything takes absolutely ages to open I would think it had frozen most of the time if it werent for the noise.

Obviously I keep my disk clean and organized, run defrag regularly, have tried Intel storage controllers, IAA(intel application accelarator) most recently which make no difference. Everything says the drive is currently working in UDMA mode 5.

Games are virtually unplayable, winamp is a bit of a struggle along with everything else really.

Dont really know what to add so I'll just wait for replies.

HELP! I'm desperate (hate those times when u have nothing to do, so u play a game but it takes 5-10 minutes to load a level)

Just because it started a year ago, doesn't mean its not the disk failing. In fact, I'd say it is the disk failing. It could be the motor in the disk wearing out.

If you've formatted and started again then it can't be any OS settings. If as you said, things have gradually got worse then its almost certainly the disk. Same thing can happen with IBM drives but it often only takes a few months for them to fail.

Is there any way of knowing for sure???

Just ran some disk checker that works in dos, and it inevitably said there was nothing wrong but what I did notice was that it was virtually silent, so for some reason its only noisy in windows.

Another thing with sandra's benchmarks was that the result I got (index 8100) was done with windows cache on to test windows performance...bypassing the cache almost doubled the rating.

yes m8 it seems your hdd is on its way out
try the diagnostic tools from seagate

http://www.seagate.com/support/disc/drivers/

Downloaded the tests, floppy based DOS set wouldnt work for some reason, just crashed the computer every time, online checkers set all was fine.

Assuming noisy drive is sign of it failing then why is it quiet in dos?

It is the performance and the fact that you formatted and the problem persisted that suggests it is a failing drive not the noise that drive is a notoriously loud hard drive

Wasnt gonna mention this but it just happened again and it might be relevant.

The last few days I keep getting a warning that my paging file is too low and windows is increasing it, most of the time it happens understandably when I have a lot going on but a few times like just now all I had open was this website.
I dont have much on startup infact only NAV as far as user apps goes and yet the task manager shows a page file usage of never less than 170MB and often as high as 330MB! Have tried letting windows manage it and setting it myself (192-384) neither has any effect.

Hate to go on but the drive wasnt noisy before this started happening and its still quiet in dos applications.