hi there windows xp is an old sytem your problem doesnt lie with the ram it lies with the pc its a old chip and ddr 2 ram hope fully youve bought decentish ram you will need 2 things to fix your problem the flash uitlity for your bios and the updated bios for your pc once the bios has been flashed it should run whats happened here is youve bought some ddr2 which was made after the mb and chip therefore your bios was designed to handle say ddr 333 and youve bought ddr2 666 so the bios doesnt reconise the ram speed although it will alter it to its own bios setting of 333 say in example, it still wont run this is also because and new part has been put into the machine i assume its 32 bit windows xp which if it is wont run ram about 4gb will only use 3.5gb max unless you use a 64bit sytem the answer to your question is simple then flash your bios in stall xp 64bit and your rocking also check that nothing else has been disturbed in the installation like cable to hdds or anything else graphics slots all that stuff in there if you post on here you mb manufaturer number code i will try and find you the link for the bios flash that you need also it may be worth checking if the new sticks work soundly using memtest which will be in your bios happy computing