If you are using Office 2007 and have client side certificates, then you can
embed a digital  signature directly in a document  based on the certificate.
If the document is altered in any way the signature is invalidated. These
digital signatures  are recognized for legal purposes in most states. You'll
need to check your local laws to find out if you are one of them.

We are satisfied with the Versioning History in SP. That shows every change the
item has undergone and who did it. This assumes you use an integrated Windows
Authentication route. I am not certain of Forms Authentication but it must be
the same. EVERYTHING is tracked in SP. Just turn on those options.

could u pls guide me where to turn on those options...

On the list that you wish to enable versioning go to Settings->List Settings. In
the General Settings column select Versioning Settings. This is where you can
turn on Content Approval (and its related Draft Item Security) as well as Item
Version History. With versioning turned on you can then begin to see any future
changes to an item.

To test, create a new item. Then go back and edit that item. You can see the
Version History by selecting an item's dropdown options and selecting Version
History, or you can see it when you display the item's DispForm.aspx page in its
toolbar. Version History will track changes made to the item by people  as well
as any actions taken on the item in a workflow. Version History also lets you
know who made the change. If a workflow  was initiated by a person it will show
their credentials or you may just see that it was modified by the System
Account, depending on the context.

I had done the similar eg. explained by u and It works.