After installing a module on Drupal 4.7 you get the error:
Fatal error: Call to undefined function: form_*() in [module_path]
This error usually occurs on the modules settings page, but also on other module pages containing forms.
Most likely cause: the module is specific for Drupal version 4.6.
The way forms are created changed completely in version 4.7. This new forms API means that all former form_* functions such as form_checkbox, form_select and form_submit, are no longer available.
Solution: You need to download a 4.7 specific version of the module in question. If the module isn't available for 4.7 you might want to try the version marked as "CVS". If that one does give the same error you should try to update it yourself. For easy forms you can use formupdater.module. An online version of this is available here:
If you succeed in converting the module you should send it to the author so that he can upload it for others to download.
You may also get this error if you use php snippets in pages or blocks that display forms. Disable such snippets before upgrading.