Some more general notes on .NET application deployment. It is actually not so bad that many people fear.
For a regular .NET application, the deployment is x-copy. Of course, you need to make sure the required version of .NET runtime and required dependent libraries exist. This is not anything unusual. For example a MacOS app written for Leopard won't work on Tiger. So is a .NET app written for 3.0 won't work on 2.0. Therefore deploying a regular .NET app is not much different from deploying other types of application.
For Excel UDF add-in, there are two additional complications. One is registration and the other is Excel version dependency.
Registration can be combined in a standard setup project. I personally prefer a simple script or let Excel to auto-register. But there are also PC release guys who actually prefer to have a proper setup.exe for all production applications, including add-ins.
Excel dependency arguably is a common issue of integration (i.e. you are integrating with another product, so by definition you may have version dependency). For example, anybody who has written HTML page before may testify the tediousness of making your web page work with all browsers. Here, instead of writing HTML that works with different browsers, we are writing C# code that works with different version of Excel.
If we exam the source of this dependency in more detail, we may find it comes from static linking to a specific interop dll (via reference). As such, we are assuming the target machine has the same interop dll. If we realize this, the solution is simple - include a copy of your PIA dll (just interop.excel and maybe VBE) along with your dll. It will work even if your user has different version of Excel installed.
In addition, from programming perspective, it is also helpful to avoid hard instantiation if possible. So for example, instead of Excel.Application obj = new MsExcel.Application(), you should try object obj = CreateInstance("Microsoft Excel Object") especially if you only use its standard COM interface.