What should be installed to convert all kinds of videos to FLV?

To convert WMV, you need to install DirectX 9 and WMV Runtime 9.
To convert MOV, MP4, 3GP, you need to install QuickTime.
To convert most of AVI, MPEG, you may download Combined Community Codec Pack (CCCP) from http://cccp-project.net and ffdshow encoder/decoder settings file from http://www.moyea.com/download/ffdshow_setup.zip, extract the ffdshow.reg from the zip file, and double click to import ffdshow settings.

Source--
Video&FLV Knowledge Originate from Moyea software