Problem with nero 5.5.9.17 plextor!

Hello guys.
I have a major problem with my cd-writer and i need help!!
I have the plextor 40/12/40A writer and while it should be burning at 40X it burns at 8X.
What should i change in order to work properly again?
Thanks:)
The nero version i use is the 5.5.9.17 one.

ps:The problem first appeared when i tried to install nero 6.0 and then uninstall it by installing again the previous version.