When I try to install any component, specifically my mtwMigrator component, I receive the following error:
* Warning! Failed to move file.
This is on a fresh install, with FTP Layer turned off, with default content installed, Joomla 1.5.14, latest PHP and my_SQL versions, Legacy mode turned on.
A very common cause of this error is due to file permissions. Joomla must be able to copy the files from your component into the components and administrator/components directories. If the system user the webserver runs as does not have write permissions to these folders, it will not be able to copy the files over.
The FTP layer is there to get around this issue. With the FTP layer on, you upload the component to the server first, then it is FTP'ed from the temporary directory to localhost. For this to work, the FTP user you specify must have permission to write to the components and administrator/components folders.
Normally, all directerories should have 755 (rwxr-xr-x) and all files 644 (rw-r--r--). If you want to set the most directories to not-writetable, you will need at least the directories listed in Help > System info > Directory Permissions. (Here you can also check which directories need to be changed, as non-writetable directories are shown in red.)
In some shared hosting environments, 755 / 644 isn't enough, as the owner of the file is not the (Web)Server User, but the FTP-User ... so either change it to 777 / 666 (not recommended, as you allow others to write your files), or get your FTP layer to work.
Another try: Joomla! does not work yet with the recent PHP 5.3. Can you use PHP 5.2? (Similar Problem in the Joomla Forum)