Just how sure are you that it was in fact installed properly?
Bought 320 Gig HD WDC and installed properly
XP SP2 allows use of gigantic hard drives, however, your BIOS must be able to hand that large drive over to Windows.
Is your system BIOS showing the correct size of your new drive? If you are unsure of how to determine this, state the make/model of your computer and someone will look up the capabilities of your machine and tell you more of what the problem may be.