Read and Write InfoPath forms from/to Sharepoint

[Jim]

I want to retrieve an Infopath form that has been published to Sharepoint
into my VB.Net web app (maybe as an XML Document?) I then want to locate a
placeholding dummy value in the document and replace it with a valid value
and store the changed (XML) Document back in Sharepoint under a new name.

Once the document is loaded I can manipulate it but I would like to use
WSS3.0 web services to do the retrieval and storage but can't find them.

Any Clues?

Thanks,

Jim