Try saving it to your transflash card then plug your bb into your computer, go to my computer and click on the drive your bb is connected to, open files, find the video and right click and save to computer or copy/paste or drag and drop to a file on your hard drive.
Do you hav one drive or two popping up when you connect to the computer. If one, ensure you have Mass Storage Mode enabled. Options>Memory>
Media Card Support "ON"
MTP "ON"
Mass Storage Mode Support "ON"
Auto enable Mass Storage Mode when Connected "YES or PROMPT"
You should now see to drives when you connect to your PC. One will say "BlackBerry" (this is the device memory) and the other will be a new drive (Media card). Open the Media Card, and you should be able to find your video.
Then follow the steps posted above to move the file from your Storm to your PC.
Were you able to save your video to your media card? Do you have a card reader? Sandisk makes a card reader called "MobileMate SD+ .. it reads SD, MSD, MMC, RS, MMC and TF ... you can pull your TrashFlash (TF) card out of your BB and slide it into the reader and then put it in the usb port on your computer and should be able to dl your video that way.
Why convert it? What type of computer do you have and what operating system is it running? Instead of converting, just download a program to play .3gp files (this is where google is your friend).