There are a few options for this.
1. I would recommend using the Microsoft Deployment Toolkit for you deployments. MDT works with WDS, but gives you much more flexibility. Using Task Sequences, you would create a Post-Install task that installs the software after the Windows deployment finishes. This method would be the least amount of work for you in the long run, since if a new version of the application is released, you simply change the MDT task to install the new version.
2. Use Audit Mode to modify your setup and re-capture the image. Audit mode is the method for setting up a default profile, used in conjunction with the setting in your unattend file. Unfortunately, if the application releases a new version, you have to either a. load the image, uninstall the old version, install the new version, and re-capture the image. Or b. Perform Off-line image servicing using DISM
3. Write a script that maps the drive and starts the install. You would have to do this every time you need this software after deployment. This requires more interaction during deployments than that other two solutions.
There are some other options to deploy software, but those 3 are all free methods. More help on deploying Windows 7 can be found on the Deliver and Deploy page of the Springboard Series on TechNet
Hope this helps,
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Windows Outreach Team - IT Pro