Try opening your backup file.
Do you have the option to create a new project and selecting a HDV, AVCHD or XDCAM sequence or are these options greyed out?
If they are all grey, your installation has in some mysterious way reverted to trial status. That requires:
3. Run Clean Script CS4
5. Repeat steps 3 & 4 twice
Otherwise follow Craig's advise.
If by backup file, you mean the auto save files, that doesn't work either.
Harm, if it has reverted to trial, should this be occurring for all projects. I can still open recently created projects.
I've found a fix for this. It doesn't seem to have anything to do with the trial version. Open the prproj file and delete the i-frame line. Save. Project will then open fine. I found the fix here: http://www.rapidshare4movies.com/adobe-premiere-pro-cs4/. Just search for the error.
Thanks for reporting back and glad you solved the issue.
I was reacting to your statement:
I've found this error is related to trial versions of Premiere.