IME, some come preformatted...some don't.
If, when you attach the drive to your system, you go to Disk Management...you will be able to see for yourself. If it's unformatted, it will appear as a drive with all Unallocated Space. If it's formatted, it will appear as a partition using either NTFS or FAT32 file system.
If unformatted, you can format it from within Disk Management.http://www.ehow.com/how_4918155_initialize...hard-drive.html