[support] drupal_get_path not found/working

Hello, I've been running Drupal for quite some time now, and I've always had a problem that I'm finally frustrated enough to try to fix. For some reason, my system does not recognize drupal_get_path. I run into errors any time I try to install a module. So, what I end up needing to do is go into the .module file and change anywhere there's drupal_get_path to be the full path to that file it's looking at. Any ideas how to fix this? Is this a drupal setting, or php? Thank you for any help you can provide. - jody