Nikon D3100 video transfer?

I started using Nikon D3100 a few months back. The only regret is I can't play videos on my computer (no problems of playing back in the camera). The videos from my camera can be transferred to the computer but when I play it, an error message pops up saying "The codec could not be found." How do I solve this? I transfer the videos using Nikon ViewNX 2 or Nikon Transfer 2 and plays them using GOM or Quicktime player. My computer runs on Windows XP

Added (1). The thing is I can't even play them on my computer. How do i solve this?

You need to buy a copy of Adobe Premiere Elements to do any editing of your video (about $110)

You can download Quicktime player if you like to view them, but to edit your scenes with others you need an editing program

As with my Canon 500D your video format, at least one of them is mov. That's an apple format that windows does not recognize without their quicktime software.

I use a program called Creator by Roxio. It's an editor that works very nicely but also contains a converter that does a nice job of maintaining the original quality as well.So, I must convert all my
mov files into wmv-9 format. That then allows me to edit in Windows.

If you're serious about video editing, an editor of this type is the way to go.