I am running AutoCAD 2007 and we need to share some files with a customer. The cusomter will insert our files into his own files as either a block or Xref file. I need to know if anyone knows if there is a way to write protect my files so the cusomter cannot modify them. Thanks

Any DWG file to be Xrefed can be also opened as a main drawing.
 
You can either distribute a DWF file instead of DWG - DWF can be underlaid as an Xref, or you could use CADVault (www.cadlock.com) to protect your data.

Vladimir,
 
Thank you for the response. If I use a DWF as an XREF can I write-protect this such that my cusomter cannot alter the DWF or do I need to use the third part product you have mentioned? Thanks.

DWF is "write-protected" by definition - it is a publishing format. You don't need anything else. You can only underlay DWF files, you cannot not import them into AutoCAD.

Vladimir,
 
I have never used DWF. If a customer of mine had a DWG file and XREF my DWF file he will not be able to write to the DWF file? What I have is a customer has designed a building and we are sending him some proprietary equipment files for him to "insert" into his DWG file. We do not want the cusomter to be able to modify our CAD file but he must be able to view our CAD file on his building CAD file. Thanks.

He will not be able to write to your DWF file. DWF files are perfect exactly for this type of jobs. Read more about DWF on www.autodesk.com/dwf