Answer
This question was answered on March 20, 2006.
Much of the information contained herein may have changed since posting.
First before anything you must make sure that a virus is not responsible for the corruption of these files. If you have an anti-virus program make sure it is up-to-date and run a compltete scan of your system.
If you have eliminated the possibility of a virus then it is possible that the files have been damaged by some other error. you can try to repair them. Follow these steps:
Open either Word or Excel.
On the File menu, click Open.
In the Open dialog box, select the file you want to open, and click the arrow next to the Open button.
Click Open and Repair, and then choose which method you want to use to recover your workbook.(You can try them one at a time until one method works)
If someone has access to your PC and has altered the files manually then they cannot be recovered. Always back up important information to another medium such as a CD.
To ensure no one can remotely access your system do this:
Right-click on the My Documents icon.
Click Properties
Click the Remote tab on the top.
Make sure the box that reads "Enable Remote Desktop on this computer" is not checked.
You may also want to add a firewall program, such as the free version of ZoneAlarm, www.zonelabs.com, to add extra security.