Also (if you have it) check gta_sa.exe too, just to be on the safe side.
Correct. Checking the box allows access, whereas unchecking the box will prevent access. After, switch off any anti-virus software you have (such as AVG, Norton, Kaprasky) and then restart your computer, make sure you switch off any anti-virus software before you test your SAMP client though. Once you've done so, fire up your SAMP client with 'Run as Administrator' by right clicking samp.exe and selecting > Run as Administrator and try the 'internet' tab on your SAMP client, you should hopefully get servers now.
Tell me how you get on.