already mentioned difference between lite & micro in quoted text in my previous post.
where did you get the key you are trying to use?different nero versions have different key.e.g.if you got nero 7 oem version with your dvd writer then you can use that key with nero 7 only.also why did you install 2 of these(assuming you meant lite & micro & not nero 7 & nero 8).
first install nero 8 from cd then follow steps in below link to find the serial no.
Nero - Tools & Utilities
after that uninstall nero 8 using Nero 8 CleanTool mentioned in step 2 of above link.now use this serial to install nero 8 lite or micro.
how about imgburn?