Did you actually tried a system restore yet?
Did you update the SAMP client to the most recent one?
Did you already Deleted GTA:SA folder and EVERYTHING along with it?
Did you re-install GTA in a folder WITHOUT anythin in it?
Did you Patch GTA to 1.0 after the re-install?
Did you check the IP of Argonath in case you mispelled something?
Did you lookup 'Argonath' into the server browser? (not the official one)
Try all these things, and when you tried them all with NO result, we see what can happen further.