I found this on Yahoo Answers, so perhaps it will help you as well (please refer to best answer chosen).
https://answersrip.com/question/index?qid=20070808043321AAC3ChL
Also, here is another link that may be useful to you:
http://office.microsoft.com/en-us/word/HP100823011033.aspx#2
TIP: Next time try to include which version of the program you are using, as well as, which Operating System you are running.
I hope this helps! Please post back if this information has helped you to resolve this issue, as it may be helpful to others that are experiencing this issue as well. :-)
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Post #2
Thank you for providing additional information. Also, you can also locate your version of Microsoft Word by clicking on the "Office Button" > "Word Options" > "Resources" > "About" Microsoft Office Word 2007 (?) - FYI - There are 8 different flavors of Word, and each may be slightly different.
I found the following Article in the Microsoft Knowledge Base which may help you to resolve this issue:
You receive a “This modification is not allowed because the document is locked” error message when you modify a document in Word 2007 or in Word 2003
http://support.microsoft.com/kb/818852/en
You may do a keyword search in the Microsoft Knowledge Base to find articles, which may help you to resolve a variety of issues yourself. The Microsoft Knowledge Base is a valuable resource and "Self-Help" option provided by Microsoft at "No Charge." The Search box is located on the right-hand side of the page found here: http://support.microsoft.com/
if this Article does not help to resolve your issue, it is possible that you may need to contact Technical Support for further assistance. It is possible that you may still be within your Warranty Support period, and then you may be able to receive further assistance at "No Charge". However, you would need to verify this with the Manufacturer of your specific product. If your product came preinstalled on your system when you purchased it, you would need to contact the Manufacturer of your computer to verify your support options with them. Otherwise, Microsoft also provides additional support options that would be available to you, however, there may be a charge for these services. For additional information, please visit: http://support.microsoft.com/
I hope you find this information helpful, and that you will post back when your issue is resolved. Best of luck! :-)