im not sure if Pentium D multipliers are overclocked, but i know my p4 northwood's was,
with an asus mobo, press delete (del) and get into the BIOS settings...
there should be a menu like "frequency control" "overclocking menu" "ai overclock"... somthing along those lines
so once you find this menu, the setting should be on auto... change it to manual, now, your standard cpu ratio is 14, so try to bring it to 14.5 and up, if its a no go, then its a locked multiplier... not a big deal
right below the ratio should be somthing like cpu/fsb ratio.... bear with me, it may be somthing else
mind that if you cant change the cpu ratio, you overclock your ram too
so from that point, raise either the cpu ratio, or the other one till you feel satisfied
bear in mind if you dont have an aftermarket cooler dont overclock much... you dont want to burn out your chip
on that note, i dont suggest this next step without an aftermarket cooler
if your menu is anything like mine, scroll down... you should hit a voltages part of the overclocking menu
by this point, you shouldve tested for stability (prime 95, and a program like speedfan to monitor heat)
when the system is no longer stable, bring the cpu voltage up slightly, go back into windows, and try again
bear in mind your Tcase (highest optimal temp) is 64C, so try not to go much above that, 70C is about the highest you'd want to get to