I recently got the below error message across all of my sites in a farm; every web part was displaying this message instead of the content. Very scary:

Web Part Error: One of the properties of the Web Part has an incorrect format. Windows SharePoint Services cannot deserialize the Web Part. Check the format of the properties and try again.

You'll quickly find the following MS KB article for the error message: KB 826786.

It boils down to SharePoint needing to work out of the WinDir\Temp and WinDir\System32\logfiles directories. Which may be a permissions issue as noted in the KB article.

This can also be caused by your virus scan software. For example, McAfee's On-Access Scanner may be blocking executables from being written to the Temp directory: SharePoint writes .dlls to temp which will be blocked, which would be blocked from functioning and cause the noted error.

So in short, if you are getting the above error message it might be a good first step to disable your virus scan and see if the message goes away, if it does work from there to readjust your virus scan settings.