You can't just use ISO image of FreeBSD 8.0-RELEASE in UNetbootin. It would not boot.
So I have read this article, which was extremely helpful.
1. Download and install UNetbootin from: http://unetbootin.sourceforge.net/
2. Download 8.0-RELEASE-i386-disc1.iso
3. Create native UNetbootin 7.0-RELEASE bootable system on your USB Flash.
4. Using any suitable program, like WinRAR or DaemonTools, copy 8.0-RELEASE-i386-disc1.iso contents to root directory of your USB Flash.
4. Boot from your USB Flash.
5. Make sure you set Release Name in Options dialog to 8.0-RELEASE.
6. Make sure you select DOS partition in installation media dialog.
5. Do install FreeBSD normally.