Im not really sure but this maybe caused your ban: If you downloaded, for example, the HL1 mod PARANOIA, by starting up the mod a file is copyed in your HL1 folder called opengl32.dll. The VAC will detect opengl32.dll as a cheat in case if you will try to play another HL1 mod (CS1.6 for example) online on the server with VAC protection.
guys ive have had some one hack into my file and hack on a game aNd got it fixed i think he did not hack hes cool funny and u should not be RETARD ABOUT IT !!!!!!!!!!!!!!!! ANS WOLFER IF YOUR RIGHT THAT THER FAGOTS