Up grading iTunes should not affect your music in anyway, but it is ALWAYS important to have backups of your valuable music.
Before making major changes, it is a good idea to make copies of your two library files - iTunes Library.itl and iTunes Music Library.xml which are in the iTunes folder.
You don't need to open iTunes to upgrade, just download the new version from the Apple site:
http://www.apple.com/itunes/download/
Save it on your PC rahther than running it over the internet.
If you want to get going quckly with your current version, drag iTunes Library.itl and iTunes Muisc Library.xml from the iTunes folder to your desktop. This will force iTunes to create an new blank library when it opens. You can then use File>>Add folder to library to add you iTunes Muisc folder to iTunes to your library. This method looses all counts, ratings and playlists.