hi
well; there are experts here who have in explained this in detail elsewhere and in different forums;
as a user, i normally install from HDD only and not from dvd; i use grub4dos;
if you are at "aware" level about multiple partitions and multi-disks, i can try to explain in simple words;
if you want to use an existing drive, you require about 40gb free space which can be partitioned into NTFS primary partition; (you can use Minitool home partition wizard for the same); copy the DVD contents or ISO extracted; what you need are two folders: BOOT and Sources; Copy the boot folder fully to the root of the partition that you created; you can copy the sources folder also fully but what you require is only two files in the folder (install.wim and boot.wim); then copy the bootmgr file to the root of the partition;
thus your new partition shall have bootmgr (file), BOOT (folder) and Sources (folder); now the next step is to start or boot the machine from this partition; this you can achieve in multiple ways:
1) if you know how to use grub4dos, then you can add a menuitem to chainloader bootmgr from that partition; it will start installing windows
2) if you dont know how to use grub (grldr) or grub4dos, the easy way is to use BOOTICE; download bootice and change the parition boot record (PBR) to vista/win7; you can use either bootice or any other partition table program to make this as an active parition; now on restarting the machine, this parition shall be loaded and windows shall start installing;
3) if you know any muliboot programs, i use SBM (smartboot manager) or PLOP; using that you can start the machine which will have an opening screen to select the partition you want to boot to; select your ntfs partition and the windows will start installing
the bottom line is the partition shall be active (or made active); should have bootmgr at the root and bcd in the BOOT folder and boot.wim and install.wim in the sources folder; then point to this partition for starting the machine;
hope you can make something out of the above
regards
neelakantan