Keeps pitching:

Checksum error in D2Client.dll
Checksum error in D2PK\patch_d2.mpq
Checksum error in D2Ex.mpq
Checksum error in patch_d2.mpq

It is also the culprit behind deleting D2Client.dll and patch_d2.mpq. No anti-virus/firewall running, Defender disabled. Multiple reinstalls to this. Downloaded client, upgraded to 1.13d.