Most probably it isn't a virus.
Worst case it can be a bug.
Or it can be caused by a workbook (maybe hidden) or module having something wrong in it.
- Go to "File - options - add-ins" (at least in 2010, I think it's similar in 2013 too), and check them.
- If you can't find any suspicious try to disable all of them to see whether the problem disappears.
- If disabling all the add-ins help then just enable one by one and check the issue to find the one causing the issue.