I have a client server application - intended for use in workgoups.
The server is a web application, clients are .NET 2.0 based desktop applications.
Is there a way to push/force the setup of the client application on all machines in the network?
Any group policy equivalent/any way to centrally manage/push an installation on a windows workgroup based network?
Not built in, that's very much the point of Active Directory.
You need an account on each machine which is a member of the Local Administrators group and has a known password, (if you spend the time setting this up once it could be useful later). With that list of usernames and passwords you could script remote installations using Sysinternals' PSEXEC for example.
There are also third party solutions where an agent is installed on each machine, like Microsoft's SMS (now System Center Configuration Manager) or the free and open source OCS Inventory NG.
You could still run a local login script on each pc, that connects to a "master" pc with the software on it and installs it from there. Checkout gpedit.msc and go to User Configuration -> Windows Settings -> Scripts (Logon/Logoff). Edit the Logon item and point to your script (batch file would be ok). The user would need admin rights locally and rights on the master pc.