Digital Signature for MS Word, Excel and PDF documents

Need a web application written that will pick a valid digital certificate on users machine and will digitally sign the document. Application will have a file upload control that will take the input and will pick a valid certificate from the broweser or machine and will sign the document. Also the program should be able to validate the signatures too. NOTE:This step is only for Word documents Next step will be to look for the signature line in MS Word 2007 or 2010 and will insert the sign or image. I need all the above steps to be done and should be totally automatic with no interaction from user.