First off, DO NOT USE WUBI. It's absolute garbage. Just trust me
Now that we've got that out of the way, on to your other questions! First off I'll say that it's generally easier to install Linux before Windows, but it can still be done the other way. If you would be okay with doing a complete re-install, I'd go this method. If not, you've got some more work. Assuming Windows is installed on a single partition, you'll need to shrink said partition using Disk Management (Control Panel -> Administrative Tools -> Computer Management -> Storage -> Disk Management) or some other partitioning tool. From here, you'll want to boot with the Ubuntu installation CD. During the partitioning stage of the installer, it will show you your options. The default option (IIRC) is to install in the free space on the drive and not touch anything else, which is what you'd want want. Blah blah blah install it (it's really straight forward). I can't remember, but I believe you have to do some configuring to Grub (the bootloader) to have Windows show up, but it may by default. I've never done this method personally, but have read articles explaining how to do so. Google will have plenty of tutorials if you search something like "Install ubuntu after windows" or whatever.