Do you mostly use this as a "desktop replacement"? Is it usually in the same place? Can you put a hole in that desktop? If you can you can put a quiet(er) fan on the underside of the hole and use that to move air through the heatsink.
Failing that, there are notebook coolers that are essentially raised decks with fans in 'em (same idea only they're not really lots quieter than the notebook fan - and putting the keyboard up on a step like that is not good for your wrists (not that the standard notebook keyboard position is any good either - but this is worse).
There's a heat-pipe based notebook cooler that helps a
bit - but I would guess that it wouldn't make much difference with a P4.
Use some compressed air to make sure there's no dust built up in the heat sink fins, and, if it were
my laptop, I'd be removing the heatsink and re-installing it with the most efficient TIC I could find. Actually, no, were it my laptop I'd be selling it on ebay or trading it in or some such and limiting my choices to Centrino laptops for a replacement, but that's not a help this time round, is it? (sorry...)
There's a
notebook forum at SPCR where a lot of this "which notebook is the quietest" gets discussed. Major reason why I looked for a Centrino this time (hadn't had a laptop for 10 years or more).