There is a "thea_path.txt" file in:
"C:\Program Files\Google SketchUp 8\Plugins\TheaExporter_files\thea_path.txt"

This file has to be deleted. When you start SU again you will be asked to select thea.exe from Thea's folder.

On Win7 the "thea_path.txt" might be located in:
"C:\Users<user>\AppData\Local\VirtualStore\Program Files\Google SketchUp 8\Plugins\TheaExporter_files\thea_path.txt"

Anyway, using installer is the recommended solution.