Download freeware Zwei-Stein Converter.
Next, download and install free 3gp codecs intended for converting files: Quicktime Alternative.
1) Launch Zwei-Stein (Click Start -> Programs -> Thugs at Bay...)
2) Press File -> Import -> Import Video Clip
3) Select type of files - "All files (*.*)"
4) Open your .3gp video clip.
5) Press File -> Export -> Video for Windows (AVI)
6) Enter filename.
7) Choose video codec. DivX, XviD or WMV9 are recommended.
8) Click OK! That's all.
NOTE: You can change several parameters before conversion
- Video Dimensions (menu Destination -> Video Format)
- Video Frequency (menu Destination -> Frames Per Second)
- Effects (menu Source -> Add Video Effect)
P/S: I haven't download the software myself, so not really sure whether it works or not. The information was taken from Gromkov.com. Do visit the link if you need more informations. There's few more ways to convert the 3gp to avi video.