Sharing Violation when saving local files - Excel 2007

Greetings earthlings. My boss has a problem with saving files in Excel 2007. Problem: Excel 2007 can only save to a local file once. Every new revision of any file must be saved with a different name. Error message: is in Norwegian. It's the equivalent to "Your changes could not be saved to [file name] because of sharing violation. Try saving to a different file." Error description: It does not seem to be LAN related, the file is attempted saved in "my documents" locally. The file doesn't need to contain macros, it will do the same if it only containts "qwerty". Excel does NOT offer to save a temporary file, like some report when having sharing violations over LAN. The user has to regularly go over the folder to delete duplicates/older versions. Setup: New Dell laptop with XP and Office 2007. LAN setup: It's connected to internet via a LAN but there's no common file server or file sharing what so ever over the network. In other words this seems to be a different case than most of the "sharing violation" threads than I've found. Has anyone experienced something like this when saving to local files? Could this be a result of i.e. the antivirus program monitoring the file?