This is weird. I have an old hp pavilion dv6 laptop, allegedly windows xp media center edition version 2002 sp3. Yesterday I inserted two different flash drives each containing .xls files as well as other files. I didn't even bother opening the excel files. I had copied them over from a win 7 pro 64 bit machine, and I am sure they were good. When I tried to eject the flash drives from the xp machine I got a message saying that a program was trying to access them. I stopped the drives again and the message went away. I am sure no files were open. When I tried to open the excel files again, I got a message from office saying they were corrupt and probably malicious. I tried to open them anyway and excel could only partially repair them, though they were still readable.

I scanned the xp laptop for viruses using malwarebytes and ms security essentials, nothing. But today I tried again with the same files and no problems. I checked the size of a good and bad file and they had the exact same number of bytes. I ran the comp command and ten bytes were different out of 70,144 bytes. So somehow the xp laptop corrupted that file by substituting bytes... If It is not a virus what could it possibly be, and how do I make sure that it doesn't happen again?
Each time I properly ejected both flash drives.

Oh, I say it is allegedly media center edition because I just ran sfc /scannow and it prompted me for a windows xp professional disk! Maybe that is the problem.. I got the laptop second hand and don't know what is going on as far as that, my computer says windows media edition. I also checked manually for this cryptolocker virus but saw no evidence of it in the registry or in %appdata%.