10GB LOST TO REFORMATT

I have recently upgraded from the WIN98 SE OS, to WIN XP PRO, i recently reformatted my pc and have decided to partition my 40GB HD in to two. the problem is after i made my partition i went into the disk managemet feature thru my manage my computer facility, as i was about to make the new partition it was only showing that i have got 30GB HD? Somewhere along the line i have lost 10GB for some unknown reason????

I would really appreciate it if someone could assist me in this matter as i am rather new to all this, i would reformatt to see if it helps but i really dont want to do this if there is any other way of recovering my 10GB lost.