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