Rotating a video

[#1]

Howdi gang 
We've been making videos of 7up jnr on phones, cameras, camcorders... whatever is available at the time... but when putting them onto the storage hard drive, some of them end up 90 degrees round the wrong way.
I tend to use the default windows media player but cannot find a way to rotate the videos. Is there a way to do it at playback time or do i need to edit the lot and re-save them?
Thanks

[#2]

Rotation can be done on-the-fly in VLC player.
Go to Tools > Effects and filters > Geometry and tick Transform or Rotate depending on how much rotation you require  
Edit: You can also batch process them in Handbrake using the <rotate> command...

[#3]

Quote from: Mav

Rotation can be done on-the-fly in VLC player.

Do you know what, that came with this PC - preinstalled.
And what do you think I uninstalled thinking "Oh i'll never use that...." 

[#4]

Sod's law that!
I have tried the rotate in VLC player and it works really well without slowing things down...
I never, ever shoot in portrait mode but my missus and daughter do and I just find it a PITA. Then again, I rarely use a phone for videos.