Excel cannot complete this task with available resources error

Our firm recently upgraded from Office 2000/2003 (some had 2000, some 2003) to Office 2007. We have been having some issues with Excel since. Some users are getting errors such as “Excel cannot complete this task with available resources. Choose less data or close other applications.” One partner told me he gets the error when he had a Word document with an excel link or is in Engagement Manager and is trying to open an excel document. Others are starting to get the message as well. Any ideas?