go into your bios settings. change your sata hdd to sata mode from achi (something like that) mode. ignore the warning messages.
download this
Vostro 1500 sata driver
extract it to floppy
boot with the xp disc. hit f6 at the beginning of it when prompt to load 3rd party drivers. insert floppy when prompt. load the driver to finish the install.
they are purposely doing this on new computer so people won't go back to xp. the xp cd can't find the driver now matter how you load it, but vista can because it has sata driver on it. intergrating the sata xp driver with nlite won't fix the problem either. it still won't find the drive. I've had this issue on 3-4 new laptops that people wanted to put xp on.