Firstly, I'd do everything Veggi said as far as your cables, cards and making sure they're in correctly.

But, did you delete the partition or reformat the hard drive before you re-installed XP? Installing XP ontop of another OS could be causing your problems.

After I made sure everything is connected properly. I'd try a clean install of XP. When you setip XP, it'll show you the partitions you have available, if there is a partition with windows on it...DELETE IT...do not continue until you're sure you've deleted the partition. Windows makes your reconfirm this like twice. Once you do that....you'll format the new partition and continue installing XP.