Code:
If EXIST("C:\program files\[a certain program]\[a certain file]")
   "Removing old program"+@CRLF
   Shell 'C:\WINDOWS\IsUninst.exe -y -a -f"C:\Program Files\[a certain program\a certain uninstall file location]\Uni114.isu"'
   "Installing new program"+@CRLF
   Shell '"\\network_path_to_current_file_server\newprogram.msi" /s'
EndIf


Quote:

second, a question about run; does run wait until the execution is done, or does the script continue directly?



Nope, RUN always returns immediately. Shell will wait, but only if the process that you run does not detach from the terminal. If it does you will need to use additional trickery to force it to wait.

All this information and more is documented in both the manual and the FAQ forum. If you couldn't find it in the manual, read the articles in the FAQ forum as they are more real world issues.