I am having the same problem.
I have a purchased copy of Photoshop CS4, nothing more.
When I installed it, apparently it installed a "trial version" of Photoshop Extended.
Needless to say I was quite confused at this and asked tech support (which was available on chat online at the time) What this was all about. He told me not to worry about it, that to let the ticker run out its course and it will go back to just regular photoshop when it was done. Loe and Behold, this is not the case.
Once the Extended trial ran out, anytime I try to open my photoshop now it prompts me for an Extended Serial Number. It will not accept my serial for good ol' Photoshop (obviously), and the prompting is all I get, no photoshop opens if I request to quit Extended Setup. Well, I don't want Extended, I didnt pay for Extended, I paid for Photoshop CS4. And my regular Photoshop CS4 Serial.
There seems to be nothing I can do to get around this issue, I have read all the "Read Me" files, and the fact that they snuck in a Trial to try and entise you to upgrade without telling you what the deal is, I think is a little shotty on thier end.
Now I am waiting on the phone for Customer Service for approx 45 minutes to try and get this issue resolved. I will let you know if anything comes of it.
Adobe, I love you, but c'mon man.
There is no problem such as you describe. If you buy a legitimate standard edition license, you can enter that serial number and the trial version will be activated as the standard edition.
Where did you get this copy of Photoshop CS4?
If not directly from Adobe, there's a chance you were scammed and your serial number isn't valid at all. Photoshop is heavily pirated.