I recently had to do a System Restore, which pretty much messed up the .NET assemblies by actually removing them from the C:\WINDOWS\assembly folder. So when I ran my projects, it would say that the System.EnterpriseService.dll could not be found. Now, I do not have admin in this particular system, so from what I read, I couldn't just drag and drop the System.EnterpriseService dlls into the C:\WINDOWS\assembly folder. So here is the other work-around.
For .NET 2.0 and below, go to C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\
The v7.0A might be another number, but all is good if you can access the bin folder and find 'gacutil.exe'.
If you find that run the command line and run:
gacutil.exe /i <path-to-System.EnterpriseService.dll>
For .NET 4.0, go beyond the bin directory and go to the NETFX 4.0 Tools directory. In the command line, run:
gacutil.exe /i <path-to-ASP.NET 4.0's System.EnterpriseService.dll>