This has happened to me four times in the past month: I go to open a PDF file and my computer tells me I don't have a program to open it. I go to my add/delete programs list, and sure enough, Adobe Reader is no longer installed. I'm on my fourth re-installation today. As weird as it sounds the program listerally vanishes in the middle of the night. I installed it and updated it one night, and then the next morning it was gone again.
I've checked for viruses but haven't found any. This has happened four times, so I'm pretty sure that I haven't been unconsciously deleting the program.