Problem: I have a Windows XP machine with Python 2.7.3 installed at ‘C:\Python27' and the scipy module installed at ‘C:\Python27\Lib\site-packages'.
I installed QGIS 1.8 recently and started exploring plugins. One of the plugins I wanted to use required the scipy module, which, as mentioned above, was already installed. However, the plugin couldn't find scipy because it was not in the ‘C:\Program Files\Quantum GIS Lisboa\apps\Python27\Lib\site-packages’ folder.
I considered installing a separate copy of scipy in ‘C:\Program Files\Quantum GIS Lisboa\apps\Python27\Lib\site-packages’ but backed away from using the scipy Windows installer when it couldn't see the QGIS python installation and didn’t give me the option to install scipy under QGIS.
Work-around: So I went into the ‘C:\Program Files\Quantum GIS Lisboa\apps\Python27\Lib’ folder and found the 'site.py' file. I opened it in a text editor and, right below the import statements, I found the text:
PREFIXES = [sys.prefix, sys.exec_prefix]
and I changed it to read:
PREFIXES = [sys.prefix, sys.exec_prefix, 'C:\Python27\Lib\site-packages']
Now the QGIS python installation is able to find my original scipy module and it seems to be working fine. Presumably, QGIS plugins will be able to take advantage of the many other [python(x,y)] modules I have installed in this directory.
Hope this is useful for others.