Transode your existing videofiles to the advanced Flash Video (FLV) format with the Riva FLV Encoder.

After selecting a source-file you can easily transcode it into Flash Video using one of the presets or your custom settings.

To view the result Riva FLV Encoder comes with the Riva FLV Player.

At present you can transcode the following formats: AVI, MPEG, Quicktime, WMV.

AVI
Cinepak Codec by Radius, Cinepak by Supermac
Intel Indeo® Video R3.2
Microsoft Video 1
Microsoft RLE
XviD
DivX 3 Lower Motion, DivX 4, DivX 5
TechSmith Screen Capture Codec (TSCC)
MPEG

MPEG-1 (VCD)
MPEG-2 (SVCD.DVD)
MPEG-4 (XviD, MP4, 3GPP)
Quicktime

Sorenson Video 3 Pro with the audio codec MP3, IMA 4:1 (nicht QDesign Standard und Qualcomm PureVoice) or wihtout audio
Exported from Quicktime Pro
Not supported: Exported from Adobe Premiere
Not supported: QDM2 Audio Codec
Windows Media Video

WMV 7, WMV 8
MPEG-4 V3
ISO MPEG-4 V1
Not supported: WMV 9
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Here is the good part that they don't tell you:

Riva FLV Encoder (normally it is used to create .flv files from .avi or .mpg, but it can do the reverse too).

Klick the Browse button in the Input tab and locate the .flv file you want to convert.

Klick the Browse button in the Output tab to select the location to save your output.

The default outputname will have the extension .flv (after all it is an FLV encoder) but just click it and change the extension to .avi or .mpg

This program worked great for me to convert an FLV file into a mpg.

It's Freeware!