Updating/editing this thread to add in a newer method using the program SARDU, which allows you to store multiple variants of windows on the USB at the same time. Old method archived in spoiler tags below. Dropbox link for SARDU: https://www.dropbox.com/s/dctpv0rc8x2nst3/SARDU_18.104.22.168.zip?dl=0 beta 3 isnt as good IMO, so heres a link to the final build of 2. Their website currently has no link for it. Be aware that during the install SARDU will offer you some third party crapware. Decline them. Sardu is NOT limited to USB flash drives. you can use DVD's, SD memory cards, USB hard drives, etc. As long as your device can boot from it, SARDU can install from it. 1. Prepare your ISO's. You'll need an ISO image or DVD of every operating system you want on the USB drive. You can also add stuff like kasperskys antivirus rescue disk, or a variety of other supported tools. 2. Rename the ISO's into the format SARDU understands, "InstallwinNAMEGOESHERE" and place them into SARDU's "ISO" folder. I've included windows 7, 8.1 and XP on the same USB stick using this method. You can only have one of each OS, but technically you could label a 32 bit copy of windows 8.1 "vista" and just remember that. 3. Add them in sardu (Beta 3 seems limited to one windows installer. avoid it for now) I started SARDU and it detected mine, since i named them in ways it understood, and told me the total size (will fit on an 8GB USB, or dual layer DVD) 4. Add anything else you want in SARDU, then click create. 5. Once done, add any other files you think you may want - drivers, antivirus, etc - and you've got a portable windows installer wherever you go. Screenshot from the SARDU website (it says boot from hard drive, in english) You simply select the windows menu, and pick the ISO you wanted. Spoiler The other thread seemed popular, so i'm adding my way of doing things. Its generally simpler and easier (one copy-paste line of text - thats it) No tweaking, no Fking around, no program installs - put the DVD in the drive, put the flash drive in the USB port and run the command. thats it. The rest of this post is purely there because people always try and do something differently, or don't bother reading the instructions properly! This does NOT work with XP, so dont ask. *****************************Above this line is all you need to make a bootable flash drive!******************************* As simple as this is, people always ask a bazillion questions. common ones are answered below. The drive letters X: and Y: need to be substituted with whatever letters your system uses. Its obvious, but some people still miss that. I've done this with NTFS and FAT32 flash drives heaps of times - havent burned a DVD for an OS in years. Your motherboard is going to need to boot from USB for this to install. Many motherboards have a "quick boot" method by pressing escape or F8 while the BIOS POST screen is still loading. Dont always assume it will show as a USB device, often it shows under the HDD menus Obviously, you're going to need a DVD in the drive, or mount the ISO via some virtual drive software, like deamon tools or Virtual clone drive If your flash drive is full, the DOS prompt will just close without copying the last files. Make sure you have enough free space to fit the contents of the ISO (a 4GB flash drive is not exactly 4GB - mine is 3.83GB after formatting, and too small for a vista SP2 install) I suggest formatting and starting with a blank flash drive - you can always add your other files back afterwards. you can do this from any windows OS - Xcopy hasnt changed in a looooong time, so it should even work in OS's as old as win95. Since this has to be clarified, i mean you can make the vista/7 bootable USB disk in older OS's. that is not the same as making bootable copies of them! and a picture guide, for those who cant live without them. 1. command line 2. format flash drive to NTFS (16GB drive) 3. Files copying 4. Open your boot menu in the BIOS, or change the boot order. My media PC allows me to hit F12 and get this: (If i had my flash drive plugged in for this photo, it would read as "USB-HDD Imation Nano" in the '+Hard Disk' menu) 5. run the vista/7 installer as normal Out of niceness, i quote Pyeti and Naelex's alternate method. This one is rather noob friendly, but requires a third party program. This method works for XP!