A recent AVG updated has caused D2Win.dll to be labeled as malware (false positive) "Trojan horse PSW.OnlineGames3.AUEH". You need to set an exception in Resident Shield and then see if you can restore the file in the Virus Vault if need be. If you have lost the file for good, after you've set an exception, a re-install - or whatever means to obtain D2Win.dll - should suffice.
"(...) but the game can basically be considered unhackable. The code is very sound. So as a word to the wise to all you hackers out there, don't waste your time trying to hack Diablo II." -Blizz, 2000