So you googled how to install Ubuntu in a flash drive and (I hope it actually happened) you found this.
There are many sites that have posted their guides on how to do this, but my post is more of a "you should really do it, it's handy" kind of thing. You can find the best (imo) step-by-step guide right on Ubuntu's download page (scroll down a bit). However, before any of that I recommend doing this.
Now what? Well now you've got a portable OS, that's what. Also, it comes in handy for fixing Windows and indeed having an OS to work with when Windows fails. It can be used to install Ubuntu on any computer too.
One last thing, make sure you can boot from the USB flash drive. You do that by accessing the BIOS on your PC when it turns on (press the indicated key for that, usually it's something like ESC or F10). You know, that weird looking thing that shows up first in every boot?
Press the correct key (ESC or F10 or ...) and BAM you're in the BIOS. Now look for the menu where there's an option to change the boot order or priority. Now give priority to USB Flash Drive over all else and you're set to boot from your Ubuntu USB stick. Congratulations.