Boot from USB via Floppy or CD

I think it might be a bit of a problem as one OS should transfer control to another OS. Apart from that the second OS should probably want to load a driver for the USB-device.  

Technically speaking it should be possible as bootable CD's implement a similar technique (First the bios emulates an floppydisk that boots into MS-DOS then MS-DOS loads a device-driver and MSCDEX to gain control over the CD-ROM).

However, bootable CD's were developed in the time when Windows 9x was still hot so there was plenty of development done there. But today MS-DOS is hardly used so it will be hard to find USB-drivers for it. (Norton Ghost is able to use external harddisks, but as far as I know they are not usable in MS-DOS as they don't have driveletters).

So I'm afraid you won't get it to work the way you want to.
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But you might try something different:

Today most PC's are able to boot directly from USB (as a BIOS-feature), so it is possible to make an external harddisk bootable and run the OS directly from the drive. MS-DOS and Linux (and even MacOS if you own an Macintosh) are able to boot perfectly from an USB-drive. MS-DOS because the BIOS is emulating an ordinary drive and MS-DOS is just to stupid to understand what's going on, Linux and MacOS by design.

However, Windows XP does NOT support booting from USB as this MSDN-Blog article explains:
"Imagine, you plug in a USB camera, the USB bus reinitializes, Windows loses access to the boot drive, and *oops* the kernel needs to page in some data and it can't." -- source: http://blogs.msdn.com/oldnewthing/archive/2004/04/15/113811.aspx
Another article on WHDC (for hardware developers) states:
"This paper focuses solely on booting from hard disk drives and CD-ROM drives for recovery and deployment purposes. Windows as it exists today is currently not optimized to run as an installed operating system from USB attached mass-storage or CD."  -- source: http://www.microsoft.com/whdc/device/storage/usb-boot.mspx

And as always, there are people who try to make the impossible work, like running Linux on an Xbox... Bart Lagerweij has created an rescue/recovery tool called BartPE: http://www.nu2.nu/pebuilder/

So I found this (non-ms) article where they managed to get Windows XP running from an USB-drive using BartPE:
http://www.informationweek.com/windows/showArticle.jhtml?articleID=177102101

Hope that Helped...

If Linux is your desired OS:
http://www.neowin.net/forum/index.php?showtopic=269145

If other...Hard to say. I know of people booting PE type environments just fine.
Full Windows? Not too sure, preleminary Googling not sounding promising.

@RiceyRice
My second OS is XP
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I know from a Dutch magazine (Computer Techniek, c't in short) that it IS possible to boot WindowsXP from a USB stick, without using something like BartPE. I want to use the tutorial to install XP on a USB-stick. But many pc's do not have support for booting from an USB-stick, especially the old ones... But I don't think it's possible too. I hope somebody knows how to do it so I will wait for a while, maybe somebody will post the solution...