Mary, what operating system is your computer? If it's Windows 7 or 8, they have the most gorgeous wallpaper themes ever.
Ana's suggestion is a good one. But run like the wind away from any searches for Christmas wallpapers or screensavers. I use those search results as examples of bad, bad websites. There are some good ones out there, but be very careful.
I believe Ryan Bliss has a couple of beautiful free Christmas wallpapers at his Digital Blasphemy website. I have a subscription there and his digital artwork is amazing.
Digital Blasphemy Free Gallery One of my favorites is a picture that JohnBoy posted here several years ago of his house after a heavy snowfall. It is gorgeous! I asked him if I could use it and he said it was OK.
EDIT: Ooo... Mary, check this out. These are single wallpapers from Microsoft. Just click on Holidays and Seasons on the left and then find a picture you like. Simply click on it to enlarge, then right click to set as your background. Nice and it will work on any operating system!
Single Microsoft Computer Wallpapers