leave all those pc wizards, pc doctors e.t.c; bios has nothing to do here, you are saying during the latest install of XP it was good, so this was the exact XP behaviour on start everything will be finr but as the days passing on and more and more programs loading on, it's performance starts degrading, that's why i said you to keep C as free as possible, i think now your PC is full of movies , high data softwares, games e.t.c; and also i think you filled C some extent more
I also had small doubt, may be your sys is infected with some spy and malwares, first tell how is the speed of system operations abd do you had internet ? and do you had any software like
VB, Visual studio e.t.c;