Installing Ubuntu without deleting the files on Windows has worked well for me.
I would set the Partitioner to use only a certain amount of space on the hard drive, say 20 GB, which is more than enough for Ubuntu. I believe Ubuntu's Partitioner uses the end of the drive, but I think you can force it to, also.
Ubuntu uses a LiveCD install enviroment, so once it boots to the CD, you click the install icon, and the installation is straightforward.
Also in the LiveCD enviroment you can backup the data you need, such as essential saves. Try to avoid picking up the virus in the backup, however.