There are ways of doing this, but migrating from any OS to another can be tedious. I have a couple of Windows machines, and a couple of Ubuntu ones, and there are differences. I would keep the old Windows machine and, if you can afford it, buy a new one with no OS, and install Ubuntu on it--you can download and burn a CD with the current Ubuntu on it (using your current Windows machine), and use that CD for the installation, then transfer files as needed. I bought a very powerful no-OS computer in France for about 300 euros, you can look online for what's currently available. I would recommend you take a look at ubuntuforums.org.....you should join the forums (free, just must choose a password), and see what you can find there. There is a huge community of Ubuntu geeks who can be extremely helpful--you will almost certainly need some help, there's a learning curve, but I found it worth the effort.
There are a few programs which work better on Windows than on WINE (the virtual Windows compiler which is a part of Ubuntu), like Picasa and TrueCrypt, so I would not junk your Windows PC, or completely overwrite it with Ubuntu.