Vista - Windows Vista Ultimate x64 Bit Uninstallation Problem

Hi to all..Well oh Vista is too heavy for 4 GB ram ..When I was rendering last night on 3D MAX wid VRay its was behaving like a crap..Anyways now I have to uninstal it..Well now am going to install my old XP x64 Bit..But the problem is that after successfull bootup and loading of drivers for XP setup, when I want to format the partision having Windows Vista Ultimate x64 Bit it says that u have to first terminate those SCSI Device before formatting the C Drive (Where Windows Vista Installed)..Well now what should I do as I have no clue whatsoever formating that C partision for new Installation of Windows XP x64 Bit..Ur input would be highly appreciated..Thanks a million..God bless ya..Cheers