My thoughts would be to down load the file, it matters not where to, then burn it to a DVD and check that it can be read by using windows explorer.
Assuming you wish to retain the OS on the smaller disc and not the larger one then Leave the smaller HD with XP on it in the pc and add the new HD as a slave (assuming its an IDE drive) or as the second channel if it's a SATA drive, reboot into XP and make a note of the new drive letters assigned to the larger drive, and decide which partition you want to install W7 onto on the new drive lets say it will be E:. Shut the mc down insert the DVD and switch on. W7 will then look at the drives and ask you which one you want to install it to tell it E: and it will install it's self there and set up a dual booting system with XP automatically with W7 as the default OS.
The alternative is to remove the smaller drive (I presume you can boot from it ok) and have the larger one only in the mc when the DVD asks you where you wish to install it to tell it D: and it will then set up a dual boot with the OS already on that disc but still with W7 as the default OS. NOTE that the partions will change so that what was D: will now become C: as far as W7 is concerned.
And you can still put the other HD in in the unlikely event that it goes pear shaped and be able to use that system.