Well, i didnt install any DVD software, and its a clean XP install. So i suppose that doesnt bring me to far.
OK, well assuming this is a clean install of Windows using a real Windows disk and not some OEM version (Dell, HP, etc) that has bloatware on it, and the driver disk did not install some type of DVD software automatically, ... and you were able to install FSX on the same hardware before, then I would check the DVD for scratches and also look into the possibility of some type of driver missing that may have to do with the DVD drive, even a chipset driver that may enable the higher IDE functions.
You mentioned you were having memory trouble. If there is something still not right in that area I can see that happening because of the amount of information being loaded from the DVD's.
Any AV or firewall running?