i got a copy of the Windows 7 x86 Build 6801 from my friend in a dvd
i tried to install it to a usb hard drive but it does not allow me to do it
the installer says that it does not support installation to hard drives connected through usb or 1394
can this be done and if yes how?

If you can;'t boot off it, it won't work.
You could install a virtual machine on it though,

but will i be able to use aero on the virtual machine
i have a aero compatible graphic card installed

Current Virtualization software won't allow AERO, as they cannot virtualize a GPU as of now.

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Hey, just bumping the thread with new life. I have build 7000 and was wondering if I could install it onto a different computer through usb. How would I check if my bios/hardware/usb can do that? Is it possible at all? How would I arrange the image files and stuff?

I tried to do the same with build 6801 back in October and It won't allow you to install it on a portable device unless it's sata no matter what computer you have. There is a way because microsoft did it at the PDC, but it can't be done through the installer. I haven't tried build 7000 yet (still downloading) but if I had to guess it probably won't either unless they have changed the Installer. If your computer is less than 3 or 4 years old It will boot from usb. you can check by going int the bios under "Boot Device Priority" and check to see if "USB" or "Removable Device" is listed there.