How to create DOS Boot USB flash drive with (X)Ubuntu

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http://askubuntu.com – I'm trying to boot Xubuntu from USB Flash Drive on my old laptop Fujitsu-Siemens but it shows "Missing Operating system. Operating System not found". Although the Flash Drive works on my current laptop. Can I make DOS USB Flash Drive with (x)Ubuntu Live CD on it? I tried various windows programs to make a bootable USB, and Xubuntu Startup-disk without success. (HowTos)