The normal route, by following the help from joomla is:

1) Install a tool

2) install patches to components (takes a long time)

3) make a migration file

4) install Joomla 1.5 and in the process upload the migration file (and hope to avoid timeouts!)

mtwMigrator

1) install joomla 1.5

2) Install and use component*

Done, thats it!

http://www.matware.com.ar/joomla/mtwmigrator-english.html

http://extensions.joomla.org/extensions/3895/details

*Please note, what your database should be UTF-8