Windows 7: mp3 noob question

Hi, I legally downloaded a Metallica song from Napster and burned it onto a 8 year old Cd-Rw i had laying around but when I tried to to burn another song onto it Windows7 media player wanted an "empty disc". What kind of disc should I use to burn my songs on so i can listen to them on my new car stereo or is what i am doing illegal and is Metallica gonna sue me for wanting to listen to the Eye of the Beholder in my car?

1. Once you burnt a song or several to a CD, that's it. Normally you cannot add to it. That's why it asks for a blank disk.
2. Nothing illegal to listen to music that you legally aquired. You just cannot use it commercially without paying royalties.

With the RW you will have to erase and rewrite the CD, along with any additional files you wish to add.

I would just use CD-R's rather than CD-RW's, and as for any legal issues, it's all a bit of a grey area imo.

ty!