"When I start up my.......laptop, I keep getting a RTNICDiag error"
A Google search tells me that RTNICDiag.exe is the Realtek NIC Diagnostic Utility. It's doesn't appear to be an essential startup program or service so you may as well try to disable it from starting every time you boot your laptop. Vista's "MSconfig" is one utility that can do this. If you're not familiar with it then here are some instructions for you:
1) Click the "Start" button, type msconfig, the program should appear at the top of the list, so click on it.
2) Click the "Startup" tab. If you can see RTNICDiag.exe and/or Realtek NIC Diagnostic Utility in the list, untick it.
3) If it doesn't appear in the Startup list, click the "Services" tab, tick the "Hide all Microsoft Services" option and untick it if found.
4) Restart your laptop and see if that error no longer appears.