How to?

I recently changed my motherboard. I used to have (and still have ) 2 hard disks. In my old rig I had Windows XP installed in the one hard disk and had the other one for saving movies, music etc. Now, in order not to lose important data from my old main disk, I formatted and installed XP in the second drive (after the new motherboard installation). So now I got 2 hard disks with each one having a Windows OS on. I backed up my old data from the old primary disk now, so I can format it freely now. Can I do it? When I go to disk management though, the format option is greyed and i cant execute it. Shall I format from the windows XP installation disk? Is it safe to do it anyway or the PC will be screwed up if it cant "see" the second OS? (which i dont use though )

you can't format it because you probably entered in the operating system of that drive.
i suggest formatting that drive from the desktop/window of your new installed OS.