Wikipedia actually has an okay write up on a lot of them.

>MS-DOS-tan is most commonly represented as a shy schoolgirl, with black or dark grey hair in pigtails decorated with "DOS"-shaped hair ties; her hair ties are usually blue, but are sometimes depicted in the colors of the MS DOS prompt logo as shown in Windows. She wears round glasses, a white button-down blouse with a light blue skirt and suspenders. She carries around a keyboard, and is usually peeking around corners.

>Windows 3.1-tan is depicted as a gothic girl with a cat named "DOS-nuko" who represents MS-DOS 6.22. Some depictions of her are cheerful or deadpan. She has long white hair with red eyes, a purple bow on her hair, a lavender gothic dress and a purple bow on her chest with a "3.1" brooch. Her cat can transform into a cat girl with a nurse outfit. This specific design is called "Grandma", but she mostly looks young. 3.1-tan is called "oobaba-sama" which means "Grandma".

>Windows 95-tan is depicted as a Yamato nadeshiko who fought a battle with the Mac-tans in the 1990's and has retired. She was aware of developments of the Internet and multimedia, but however, she is unfamiliar with developments after Windows 95 such as the USB. She has long brown or orange hair, brown or pink eyes, and a bow of the Windows logo, a kimono of the hana256.bmp wallpaper in the Japanese version of Windows 95 and black sandals. She is often depicted as a motherly and kind woman.

>Windows 98-tan is a shy girl with an appearance of a middle school-aged person. She also has a mecha box, just like Windows 98SE-tan. She is 98SE-tan's sister. She has a dark history notebook. She has a blue bob cut, a golden 98 hair clip, blue eyes, a sleeveless white shirt and a blue or black skirt, blue or black thigh highs and brown or black shoes. She also has blue or black gloves.

>Windows 98SE-tan is depicted as a girl with an appearance of a middle-school-aged person who is prone to aggression and holds a can opener with her. She is a fan of professional wrestling. She has a mecha box, which is inspired by Vulcan from Zatch Bell. She has periwinkle or blue hair, a side ponytail with a cherry shaped hair tie. She has a green or teal sailor uniform.

>Windows Me-tan is a clumsy girl and tries her best in everything, but as a result, she always fails, but is always cheerful. She is the first OS-tan in the Futaba Channel, being created on August 6, 2003. She has mint green braids with an ahoge, a blue maid dress, with an orange or red bow on the chest with the Windows Me system restore icon. She often carries a leek with her and her favorite food is a Japanese roll cake. 2K-tan is her guardian.

Of course there are a lot more of them for the later iterations of Windows. Some people have made some for Unix and Linux. And while it's harder to find, some people have made them for more obscure operating systems like BeOS, Haiku (spun off from BeOS...it's actually fun to use), AROS Research Operating System and of course things like iOS and Android. On Japanese internet you can find them for even weirder operating systems.

FYI, if you ever feel like trying out some really obscure OS, this website has a great selection of ISOs you can grab and attempt to run in a VM! https://archiveos.org/