you have to partition/format it
the directions to do that in for windows xp is:
Click Start > Control Panel > Administrative Tools > Computer Management, and choose Disk Management from the tree at left
Your new drive should appear, with a black bar indicating it isn't partitioned. Right-click the bar, and choose New Partition to launch the New Partition wizard.
Installed drives and their partition schemes appear in the Disk Management window's lower-right corner. Click Next, and check that Primary Partition is selected; click Next again, to the Specify Partition Size screen (don't change the partition size in the "Partition size in MB" field); and click Next to advance to another screen, on which "Assign the following drive letter" should be selected. Click Next yet again (to the Format Partition screen), and ensure that "Format this partition with the following settings" is selected and that the "File system" drop-down reads "NTFS." Click Next a final time, hit Finish, and formatting begins.http://computershopper.com/hard-drives-burners/howto/install-a-new-hard-drive-in-five-easy-steps
Vista and Windows 7 is easier, just right click the hard drive and click format