Viruses could still be an issue.
If they were an issue on most systems other than Windows.
Yes, I realise I'm going to start a flame war saying this. But the reason that Windows has so many viruses and other nasties isn't just because it's so popular. It's also because its security (certainly up to Vista, anyway), has been fundamentally more lax than Unix-derived systems.
If it was just popularity, then if you divided by the installed user base, then you'd expect to hear about a lot of viruses for Windows, but still a fair few for Mac OS X, GNU/Linux systems, *BSD systems, etc. But you don't. There essentially are none that exist in the wild.
(Part of this is also down to the fact that Windows users are in general more stupid than people using GNU/Linux, *BSD etc., so are more likely to be tricked into running a virus-ridden program.)
So, don't worry about viruses, but know that it's still possible (even just from the CD -- some viruses could easily be made to look for any drives attached -- including your internal drive.)
Regarding your mp3 player and satnav. What software do you use for them? iPods (latest models) are a bit annoying because Apple encrypts their song databases. But practically any other brand (especially the ones you just drag music into like a flash drive) should work fine.
If the Satnav needs special Windows software, then check the app database for Wine. See if that software runs with Wine:
http://appdb.winehq.org/
(Wine is very easy to install with the Synaptic package manager, though it's probably on already. You'll need Ubuntu on your hard drive to download and install things though.)
Regarding Firefox: yes I agree, that is a stupid mistake on their part. Largely because there was a big issue with fsync, causing huge slowdowns.