I KNOW SP2 supports SATA natively...not sure about SP1 and before... Anyway...you'll have to format the drive. To do this, do the following:
Start > Right click "My Computer" > Manage > Find 'Disk Management' on the left side > Find the new 400GB drive in the bottom half of the screen (it should say something about unallocated space) > right click > format (or create partition, I forget which). Pick Full Format as NTFS. Wait for it to finish, and you should have a newly formatted drive =).
Steps above are the same as in Vista, I believe as well. Only difference is that instead of "My Computer" its just called "Computer."