You may be caught in a Catch-22. The HDD was partitioned and formattted, then old XP installed in an environment that did not support HDD's over 137 GB (that is, did not support 48-bit LBA). Now, even though you have upgraded XP to SP2, it will NOT automatically rearrange your drive.
Now, IF you simply want to create a new partition on the HDD that will allow you to use the rest of it as a different "drive", you should be able to do that in XP's Disk Manager. Can't remember right now exactly how to get there, but it is through Control Panel and then Administration of storage devices. When you get there it will show you the existing drive and then another empty space that you can right-click on to perform operations - like, create a new partition, then format it, then name it.
However, if what you really want is to expand your C: drive to be the full 400 GB, we have a problem. Windows can expand certain partitions into unused spcae, but it will NOT do that if the drive in question is your boot drive, or if it contains the Page File. I understand there are third-party tools that WILL do this, and maybe even Samsung's tools can help.