|Watch the POST screens at start up to verify that the BIOS is properly identifying the drive by both the model number, and the FULL capacity. |
If it isn't then your BIOS is NOT 48 bit LBA compliant. That means your BIOS can't properly use any hard drive larger than 137GB.
Another problem can be if you have re-install WinXP and used a CD that doesn't have any service packs integrated into it.
Post back with those results.