Windows 7: How to make your external HD bootable

Of course! Don't worry about BIOS configurations.
My main problem is time related. I use that disk for image restoring with Acronis True Image and also I've most of the tools I use when I'm reparing, diagnosing or installaing PCs. So, before I do this, I've to make sure that I've backuped all my data. Some loss of data may be equal to lots of hours lost. :S
But, I'm doing it right now... I'll give you the news in a pair of hours.

I don't know if I have understood your answer but I think that you want to install windows seven withouth a DVD.

It's easy, you don't have to do nothing, and I can't understand why people burn a DVD to install windows 7 or Microsoft recommends it.

Windows 7 copy all the necessary files onto your hard disk before reset the system. When windows 7 reset your system, it never reads dvd drive again!!

I downloaded the ISO, I loaded it in Daemoon Tools, and windows installed itself without any problems.

Windows Xp could make this too, but you had to have care, because you had to check a stick(I don't know if this is the word).

If your question was, that your computer doesn't load any SO (for example you had a virus). I don't know if you can make a Bootable-Usb-Disk to repair it. I think that this is a special function on some BIOS

Tip: make your PDC 2008 (or any) USB hard drive a bootable Windows 7 install disk - istartedsomething

Guide I used for my external HD, works rather well

Well guys, good news.
I had no time to backup all my files from my external HDD, so I decided to apply the easiest and fastest method... and it works! ^_^
You don't even have to backup your files or change the dimensions of your partitions. Just right click on Computer and select Manage. Go to Disk Management and set the partition of your external hard disk as active. Next, copy all the files from your Windows 7 disk image into the partition you've set active. An fast way to do that is, if you use WinRAR, open the disk image with it and extract all the files to your external hard drive.
That's all! I don't even had the need to reconfigure the USB emulation on the BIOS. Just use your boot selector when your machine is in the POST, choose your external hard disk from the list and the rest goes like magic. ^_^
Tested on Asus P5N32-E SLI, Acer Aspire 4520, Gigabyte GA-945GCM-S2, Biostar P4M890-M7 and Biostar 945GC-M7 TE. I want to test it later with an ASRock motherboard because this motherboard's brand was always a problem when booting USB devices. :S
Ah! I almost forgot: this method also applies to pen drives. I've tested it with my cheap Kingston DataTraveler 101 with 8 GB. of capacity on the same machines I've mentioned before.

Hope it helps!
Sayonara! ^_^

Something I did was.. I read a tutorial on "Windows PE" (Preinstallation Environment).. I downloaded the required tools to set me up a disc with vista image file support, and some other things, and burned it to a CD (you could also just put it on a thumb drive.. incase you don't have one big enough to fit Windows 7...)

It takes some time to setup, but there are a bunch of sites with step by step command line walkthroughs. It's pretty simple. What it essentially is.. it's like a new age DOS equivilant.. or like a linux live CD.. you have access to a bunch of tools (+ whatever else you put on the CD) to help you setup/maintain your files.

I essentially extracted the ISO into a folder on my D drive, made a CD of Windows PE with all the necesary tools, booted from it, and then ran the Windows 7 install from the command line in Windows PE. It installed flawlessly. And now I have a "new age bootdisk" so to speak that supports NTFS, among other things.

If you want the 64bit Windows 7, I think you need the 64 bit Windows PE, or else you can't run the windows installation.

The Windows PE method is a nice one. I use that method since a couple of years ago. I used to use my own Windows PE with my own customizations that fits my needs.
The unique problem with that method is that you've to spend a good bunch of hours reading tutorials and testing things before getting things running the way you need or like. Also, you're forced to use an optical drive or, if you want to boot it from a USB drive, you need to read a new big amount of documentation and tutorials to get it working and with the posibility that you can get some errors too.

We can say that the Windows PE method is flexible, capable to add new tools that can help you on your work, but is hard to get, needing lot of hard work and testing.
The booteable USB method is very very fast, letting you to prepare a disk that can be used for installation in just a few clicks and minutes and don't even using a single CD or DVD, but it doesn't let you do another thing apart from running the Windows 7 installation in a very high speed in most cases.

Is up to you if you can spend some time getting a nice toolkit or if you just need to install Windows fast and easy.

Hope it help you to decide what you have to do.
Sayonara! ^_^