I have downloaded some photos and videos from my phone to my computer. However, all the videos are sideways on and I can't rotate them. They are downloaded into Windows Live Photo Gallery.
Can anyone tell me how to rotate the videos though 45 degrees, please.
45 degrees??? Ruddy 'eck! Maths must have changed a lot since I used to teach it. There used to be 90 degrees in a right angle! You've hopefully got VLC on your computer. (If not, why not? It's about a zillion times better than anything from Microsoft): https://www.videolan.org/vlc/download-windows.en-GB.html That will enable you to easily rotate any...
Alternatively you could try one of the many online services which offers to do the job for you (but I can't actually vouch for any of them myself):
https://videorotate.com/