Can I create a bootable USB on a MAC which I can use on a toshiba laptop which is basically stuffed ?
You want to create an Ubuntu live USB that boots on a non-working PC. The good news is, PC's and Macs are equally capable of this. There's no impediment to creating the flash drive for the PC just because you're doing it on a Mac.
Assuming this Mac is of relatively recent vintage, and thus has an Intel processor, the ISO images you'd write to USB for use on a Mac would be the same or similar. But even if they weren't, the architecture of an ISO image being written to a USB flash drive would not have to be the same as the architecture of the machine writing it.
Writing an Ubuntu Live USB on a Mac, to Use on a PC
Assuming your Mac is running OS X, you can write the ISO image to the USB flash drive using this method or one of the methods presented here. It's advisable first to check the ISO's MD5SUM (see below for why this may be a particularly good idea in your situation).
So here's a summary of one pretty easy way, from beginning to end:
Can it be used on the non-working / stuffed / borked laptop?
The bad news is that your laptop could be broken in ways that keep it from booting at all, even from a USB flash drive.
But if the OS is installed on your laptop (here, Windows) is what's broken, that's fine. You can boot from an Ubuntu live USB. With the live system, you can use the computer (but you cannot save any files permanently to the live system, you must use a separate volume for that). You can also access files in the Windows system (which is handy, as it lets you recover documents and other important files).
You can also install Ubuntu. If you don't plan to attempt repairing your Windows system, the best way is usually to offload all the important files from it, then install Ubuntu to use the entire disk (which will overwrite it, so you must have your documents elsewhere, preferably in two places so they're backed up).
Well I finally found an answer that allows me to create a bootable USB of Ubuntu..... http://renevanbelzen.wordpress.com/2009/10/14/creating-a-bootable-usb-stick-with-mac-os-x-in-10-easy-steps/
In effect this allows you to mount an image on the USB and off you go. It uses Terminal in OS X to unmount the USB, copy the .ISO file renamed to .DMG using the dd (convert and copy) and unmount it again. Amazing it worked. My UNIX is very old so that might not have worked. The throwback of dd to JCL was amazing.... No reason it should not work for a Windows.ISO!If you really wanted that!!!! Ubuntu is now installed on my PC at last and death to windows8 which someone calls an operating system! Hurray.