you may have installed windows XP right over the old windows XP. i would reinstall windows XP, but this time delete the partition that winXP was on and than create the same partition again. you can do all this with just using the winXP CD.
or you can take the hard disk to another windows XP machine and format it by right-clicking on the drive and selecting format or use administrative tools>> computer management>> storage.
im not sure exactly the names but that is about right i think.
make sure you check the contents of the drive before you format, otherwise you will have a bigger headache to deal with. if you do not have NTFS partitions you can use any windows operating system to format. and since it already has been formatted you can save a lot of time by using the quick format option. if you are using the winXP CD method of recreating the partition make sure you delete the correct one and DO NOT use the quick format option because you have created a new partition.