After Effects is an amazing program. That's not particularly newsworthy.
But when using third party plug ins, it's not uncommon for a "demo" version watermark to reappear - even after you've paid your money and activated the plugin. In the past two weeks, I've experience this problem with Magic Bullet Looks and Zaxwerks 3D Invigorator Classic.
I'm not sure if this is a Windows issue or an After Effects issue or a plugin issue and I don't really care. Broken software is a real momentum-buster when you're trying to get something done. And when aren't we?
Anyway, here's a simple fix to try before spending hours attempting other remedies.
The problem can be found in the After Effects preference file. You can force the application to rebuild it by pressing and holding down Cmd-Opt-Shift (mac)/ Ctrl-Alt-Shift (win) immediately after you've clicked the AE icon.
I've found this solution is quick and often corrects the watermark problem.
If this doesn't help don't hesitate to involve the software support staff. I've worked with the folks at Magic Bullet Looks and Zaxwerks and both were friendly and responsive.
Also please give this blog post a look. Jonas Hummelstrand at General Specialist.com gives a thorough overview of troubleshooting After Effects issues.