Asus P5Q Deluxe memory problems

Hello this is my first post here so if i've posted this in the wrong place i apologize

I have a custom built pc running windows xp pro sp3 with an asus p5q deluxe motherboard with two 1 gig sticks of corsair xms2 memory 800mhz (installed in the yellow slots). This has ran fine for a couple of years, but has slowed down a bit when a lot of programs are open so i bought 2 more sticks of the same memory and installed them in the black memory slots and now the pc wont boot. It will boot fine with 2 sticks, but not 4. I have altered the voltage from 1.8 to 1.9v (which what it says on the sticker on the memory and the timings to that of the sticker 5-5-5-18, but no joy.
I have also removed the old memory and put in the new memory in its place and it doesnt post with the new memory. The only difference i can see with the new memory is that its a different version no, but would this stop it from posting?

Im now stuck as to what to do. Does anyone have any ideas?

Thanks very much

Pc specs

asus p5q deluxe
Intel core 2 duo 3ghz
geforce 9800gt
windows xp sp3
soundblaster xfi xtreme music
600w power supply
2 sata drives
 
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