Might be worth checking on answers.microsoft.com - I had the problem with the start button not working a while ago and I seem to remember there was some mention of dropbox causing a problem so may be worth taking a look. You may also find other people on there who are having similar Excel problems to yours since the latest update.

I was one of the lucky ones and managed to roll back my Windows 10 install to Windows 8 when MS couldn't seem to come up with a sensible solution to the start button issue!