You should be able to install the upgrade from disk without doing a clean
install. Having said that, it is still best to backup your data to an
external drive, if you have one. Should something unforeseen happen you will
have your data safe.
We don't know how old your computer is - the older the more likely you might
have issues to overcome but you have used the Vista Upgrade Advisor, which
should point you in the direction for more important issues to resolve. You
can help yourself further by uninstalling any programs that might cause
problems during installation. These would be your third-party security
software - anti-virus, anti-spyware, firewall that are not Vista ready.
Also, uninstall any software shown to have issues with Vista by the Vista
Upgrade Advisor. You can always install them after Vista is loaded and try
them to see if the will function, or update the software to a Vista version,
should one be available. I had to update Adobe Photoshop Elements 4.0 to
5.0, and I just purchased 4.0 about 8 months ago!
One other thing I did that few talk about is to update all my hardware
drivers. I have a subscription to
www.driveragent.com so I can download
drivers from them for a year to update all the computers in the house. Now,
you don't need a subscription to have them scan your computer, which I
recommend you do. Once you find out which drivers are considered "bad" you
can go to the hardware device maker's website to download and install the
latest WinXP drivers at no cost. Doing this will put you in the best
possible position for installation of Vista. Then your install should be
easy once you get it started. You just let it do its thing loading,
rebooting, loading, rebooting. It will, towards the end, appear that it has
hung - you will see a black blank screen for what seemed like 5 to 10
minutes. Just leave it alone and wait. If everything went well it will
finish. Be patient.