Actually, the easiest method is to use the Windows7 USB/DVD Tool from Microsoft. Put your thumb drive in a USB port, run the tool, point it to where your ISO file is, and it will format the drive and make it bootable.
Now that's the first part. The next step you have to do is to reboot your system and carefully look for the key you need to press to change your boot options (usually, it's the ESC key, but yours may differ- keep rebooting until you figure it out). Once you've pressed the magic key, select HDD and then select the USB drive you used.
That's it!
PS- You can also change your boot order in the BIOS, but why go through that trouble when you only need to do it this one time?
PPS- Since you're probably doing this to install an OS, when the PC reboots the first time during setup, remove the USB drive.