Asuming you have Windows 7 and you have a MBR partition HDD it is very easy to boot from your ISO in a secondary partition:
Make a folder Isos in the root of your secondary partition, copy there your bootable ISO.
Copy to your notepad:
[boot loader]
timeout=5
[operating systems]
c:\grldr="Grubfor2"
and save it as boot.ini in your Isos folder
also put there a copy of grldr (from grub for dos)
Copy to your notepad:
color white/blue black/light-gray
timeout 30
default 0
title Run Windows 7 (Loads BOOTMGR)\n Find and load Windows 7
find --set-root /bootmgr
chainloader /bootmgr
title Run Win7PE Lite\nRun Win7PE from HDD
##ISO file must be contiguous on disk in this case, use Wincontig
find --set-root /Isos
map /Isos/YOUR-ISO.iso (0xFF)
map --hook
chainloader (0xFF)
title Grub4Dos Commandline
commandline
title Reboot
reboot
title Turn OFF
halt
And save it as menu.lst in your Isos folder
Boot from your USB drive in your virtual environment and move to the root of the very first partition (boot partition where boot manager resides) all files from your Isos folder excluding YOUR-ISO.iso and that's all. Reboot and now you will have a secondary option to boot from Grubfordos.
Of curse you need to sustitute YOUR-ISO.iso with the name of your booting iso aditionaly you could use any name for the Isos folder but you need to change that name in the menu.lst file in acordance to the new name.
If you want to add aditional bootable isos just put them in the Isos folder and add them to the menu.lst
Best Regards