So I as understand it you have XP Pro install on your C: drive and XP home installed on your D: drive?
You can still format c: even if you partioned your hard drive. It will not damange any files remaining on the d: drive
You can do this by booting off the XP install cd and when choosing the partition to install on choose the option to format it before installing the OS.
If you want to get both C: and D: formatted back into one large drives you will need to use a partitioning program... either FDISK or a third party one like patition magic.
Post again if you need clarity