Can't test it right now, but I think this should work:
SELECT c.LastName as Customer ,
e.LastName as SalesPerson,
s.ProductID,
p.Name as ProductName,
SUM(s.Quantity) as quantity,
SUM(p.Price * s.Quantity) as amount
FROM dbo.Sales s,
JOIN dbo.Customers c
ON c.CustomerID = s.CustomerID
JOIN dbo.Employees e
ON e.EmployeeID = s.SalesPersonID
JOIN dbo.Products p
ON p.ProductID = s.ProductID
AND p.Name like 'Paint%'
JOIN (SELECT ProductID,
AVG(Quantity) as avg_Quantity
FROM dbo.Sales) s2
ON s2.ProductID = s.ProductID
GROUP BY c.LastName , e.LastName , s.ProductID, p.Name
HAVING sum(s.Quantity) < AVG(s2.avg_Quantity)