ok, well the only thing i can think of right now is that the hard drive you have with the OS on it is formaed as a FAT32 Drive and the other Hard drive is an NTFS drive.
The only way to see the second hard drive is to format your current OS drive to NTFS because you can not see an NTFS drive if you have FAT, FAT16, or FAT32 as your main drive, it will only work if you have NTFS as your main and the FAT was your secondary.
But in your case to check to see what your drives are XP open My Computer and click your C: drive, and see what type of file system it is over in the left info bar.
If it is FAT32 you can do one of the following:
1: Format the main drive to NTFS and lose all your info
2: Format the second drive as FAT32, which i do not reccomend.
If it is the way i think it is, you are better off backing up your files from the first disk, and formating it as NTFS to match the second one.
P4 2.5 Ghz/socket 478/400FSB/512L2
512 megs PC800 Rambus
ABIT TH7II-Raid mobo
128MB Nvidia FX 5200 Home Theatre w/TV out and Remote
80 Gig Western Digital/8mb Cache/7200 RPM\'s
20 gig Fujitsu/2mb Cache/5400 RPM\'s