Private Sub Command1_Click()
 Dim x As Integer
 Dim Y As String
 Dim I As Long
 Dim C As Boolean
 Dim text0 As String
 If (Combo1.Text = "十六进制转十进制") Then
    Text1 = UCase(Text1)
    If Len(Text1) <> 0 Then
      For I = 1 To Len(Text1)
         text0 = Mid(Text1, Len(Text1) + 1 - I, 1)
         If C = Not (text0 Like "*[!0-9A-Fa-f]*") Then
            x = MsgBox("请输入十六进制数！", vbOK)
            If x = vbOK Then
               Cancel = -1
            End If
            Text1 = ""
            Text3 = ""
         End If
      Next I
    AA = HEX_to_DEC(Text1)
    Text3 = Val(AA)
    End If
   
   ElseIf (Combo1.Text = "十进制转十六进制") Then

         If C = Not (text0 Like "*[!0-9A-Fa-f]*") Then
修改如下试试：
         If Not (text0 Like "*[!0-9A-Fa-f]*") Then