Thanks for the reply.
Dropbox isn't open and I don't have it installed. The error message says 'such as dropbox' which suggests it just assumes there is something else accessing the file. I haven't used dropbox or moved launchbox. All previous updates were completed fine with no issues and this is the first update I've ever experienced this problem. From other posts, it looks like it's not just me who has had this issue.
Looks like i won't be able to update until I find a workaround.....