first of all a quick terminology check. - I assume you partitioned your drive and you created a 25 gigabyte partition where you intend to install Ubuntu.
If you want a swap partition, then now is a good time, before you install, to delete that partition you made and create a new 25GB minus your desired swap partition size.
A good swap size is somewhat a matter of opinion these days, and depends on the amount of RAM in your computer. As a very rough rule, go with the same size swap as your amount of RAM, assuming you have at least 2GB or RAM (as you are running Windows 7, I will assume you do.
So, make a 23GB parition and make another 2GB partition for swap.
Don't bother formatting it with NTFS this time, as you will need a different format for your Ubuntu install. In fact you are probably better off just leaving the space unpartitioned and allowing the Ubuntu installer to deal with it.
Your Ubuntu installation should go well, and if you install the boot loader (GRUB) to the Master boot record, you should, when Ubuntu has finished installing, be able to dual boot into Windows 7 and Ubuntu. That is the way it is designed to work.
However, things can go wrong, and as there are so many variables and possibilities, the best advice I feel that I can offer you is to find somebody who knows how to recover an non-bootable system and who has the tools to do it. If you can get them to hold your hand as it were, during the installation, you will be fine.
If you decide to go it alone, then do the installation beside another computer that has access to the Internet, so that if at any point during the installation you feel uncomfortable about something, you can search for help. Be careful when choosing the partitions to install to.
Something you could do, if you are up for it, is to install virtualbox http://www.virtualbox.org on your windows 7 OS and create a virtual machine to practice installing Ubuntu on a virtual hard disk. This may allow you to become more comfortable with the installation procedure before you go ahead and install on your internal HD.
It goes without saying that you should backup your entire Windows 7 install before you proceed.
At the very least, (as to be honest, if you are wary of installing Ubuntu, a full backup that would allow you to restore to a bootable system may be somewhat impractical for you) make a copy on an external USB memory or hard drive of all your work and documents, and ensure you have your original Windows 7 restore media and installation media for the programs you have installed, so that in the case of absolute disaster, you can factory install your Windows 7 and then restore your applications and your personal files.
I won't address the issue of the live USB, as you don't actually ask a question there, however, if you are asking for help with that, I would suggest you refrain from asking about two issues in the one thread.