loud squealling noise when i start pc ?

[hardware_newb]

hi need some help to get my audio problem fixed,i just wrote a long post just to find out i wasent loggede in when i tryed to post it.. dohh

well my prob is i only get "white noise" instedt of sound when i start my pc,but if i reboot 2-3 times its allmost normal again,when i say allmost is it couse when i try to game (call of duty2) i loose ingame sound after 10-15 min.i get a major lag and then the sound is gone from my point of view,i can still hear all the other players footsteps,guns ect but if i spectate a player then the sound is gone again.i then have to go out in game options put it on medium sound quality apply then back again and put it on high,but then it only works 10 min
on my old rig there wasent any prob but got some new parts and put em together and now i got this prob..
specs:
asus p5k-e
core 2 due e6600
ram corsair 2 x 1024 pc 6400 ddr2 800 mhz
gfx inno3d 8800gtx 768mb
psu c.innovation 520 ( know i need a bigger psu )
sound xfi Sb extreme gamer pro series
windows xp sp2
when i installed the things for mobo (there was a quick inst. so it did it all )
but i got audio drivers inst. also + didnt disbale my onboard soundcard before i put my xfi in and loadet all the drivers for it (dumb i know,but im a newb)
i disabled onboard sound,removed hd audio drivers (i think i got it all)and reinst. xfi+ looked in system info and theres no hardware conflickt or anything but i still got this problem.
is it old drivers cousing this trouble? or is it somthing with my pci slot sharing?
i really would like to avoid a re. of windows .. hilfe plz
can any1 help me what to do sry for all the stupid questions and long post but im completely lost in this pc world

i just wonder why it work normal sometimes.. shouldnt it be a constant prob?
sry for my poor eng