void UserDatabaseLoad()
{    
    User *NewUserNode;


    ifstream infile;
    infile.open("UserDatabase.dat", ios::in);
    if(infile)
    {
    
 while(!infile.eof())
 {
    
   NewUserNode = new User;
   

   if(NewUserNode != NULL)
   {
   infile.get(NewUserNode->UserName, 30);
   infile.ignore('\n');
   infile.get(NewUserNode->UserPass, 30);
   infile.ignore('\n');
   
   CreateNode(NewUserNode);
   }
   else
       cout<<"Error Loading UserDatabase.dat"<<endl;
 }
 infile.close();
 SysLogon();

    }
    else
    cout<<"Unable to load Userdatabase.doc"<<endl;
}

void CreateNode(User *NewUserNode) // Creates and inserts node into Linked Lists
{
    User *Hold;
    NewUserNode->UserNext = NULL;
    NewUserNode->UserPrev = NULL;

    if(UserHead == NULL) // If the list is empty
    {
 UserHead = NewUserNode; // creates a new node and assigns it to head
    }
    else // if list is NOT empty
    {
 UserHead = UserCurrent; //current node is now head node
 UserCurrent = NewUserNode; //head node takes the value of the new node

 UserHead->UserPrev = UserCurrent; // assigns the pointer to the previous node to the 'current node'
 UserCurrent->UserNext = UserHead; // current node's next pointer is assigned to the newly created head node
 
 if (UserCurrent->UserPrev == NULL) // if there is only 2 items in the list they are also linked together
 {
     UserHead->UserNext = UserCurrent;
     UserCurrent->UserPrev = UserHead;
 }

 else // if there is more then two in the list
 {
     Hold = UserCurrent;
     while(UserCurrent->UserPrev != NULL) // cycles through list until the end is reached
     {
   UserCurrent = UserCurrent->UserPrev;
     }

     UserHead->UserNext = UserCurrent; // assigns the end of the list node's previous pointer to the head pointer
     UserCurrent->UserPrev = UserHead; // links the head pointer to the end of the list
     UserCurrent = Hold; // reassigns the current Node

 }
 
    }

}

void SysLogon()
{
    char TempUserName[30];    
    char TempUserPass[30];
    int a;

    system("cls");
    
    UserCurrent=UserHead; // assigns the head node to current node.

    cout<<"Welcome to Vextor Banking Solutions' Banking System V. 1.00 (Test) (c) 2004 Vextor Inc. TM B-diddy Industries Rio Rancho NM\n";
    cout<<"Username:";
    cin.get(TempUserName, 30);
    cin.ignore(80, '\n');
    cout<<"\nEnter Password\nPassword: "<<endl;
    cin.get(TempUserPass, 30);
    cin.ignore(80, '\n');

    for(a=1; a<=UserCurrent->UserNodeCount; a++) //creates a loop based off number of nodes
    {
 if(strcmp(UserCurrent->UserName, TempUserName) == 0) // compares current node's username info to user entered info
 {
     
     if(strcmp(UserCurrent->UserPass, TempUserPass) == 0) // compares passwords
     {
   cout<<"Welcome to Vextor Banking\n"<<endl;
       MainMenu();
     }
     else
     {
   cout<<"System Login Failed: Invalid Password\n"<<endl;
   SysLogon();
     }
 }
 else
 {
 UserCurrent=UserCurrent->UserPrev; // cycles to next node to verify information again
 }
    }
    cout<<"No such User found\n";
    
}

UserHead = UserCurrent; //Not sure whta UserCurrent is, but why make it the head?
UserCurrent = NewUserNode; //Did you mean for UserCurrent to be Tail? Point to the last node in the list?

UserHead->UserPrev = UserCurrent;  //There is nothing before the head, head comes first therefore Head->Previous = NULL
UserCurrent->UserNext = UserHead; //This would make the UserCurrent point to the head

size = s > 0 && s < 1000 ? s : 10;

return a > b ? a : b;

if(a>b)
  return a;
else
  return b;