I found my roommates tape on my computer, do i tell him about it?

My roommate and I are best friends and having been living together for about 2 years now. We both either have or have had long term girl friends. This past week i was getting my computer ready to reformat and for some reason i ran accross a video that i did not recognize. I click on it and much to my surprise it was a video of my roommate and his old girlfriend having . Why is it on my computer? Knowing my roommates personality he would never risk putting something like this on my computer. However, it could have been his old girlfriend, or they could have been watching it one day and not deleted it properly. Nevertheless, do ask my roommate about it? I don't want to taint our friendship, however, im just thinking if one of my friends found that of me i might want to know about it. Let me know what you think.