Well, it's not pretty, but it's the only way I could get it to work:

var q3 = (from prop in ProductProperty.All()
     join field in ProductDefinitionField.All() on prop.ProductDefinitionFieldId equals field.ProductDefinitionFieldId
     where field.Name == "Medium" && prop.Value == ((!string.IsNullOrEmpty(Param2) && Param2 != "Any") ? Param2 : prop.Value)
     select prop.ProductId).ToList();
 
q3.Add(-1); // Prevents NHibernate from throwing an exception if list is empty
 
var q4 = (from prop in ProductProperty.All()
     join field in ProductDefinitionField.All() on prop.ProductDefinitionFieldId equals field.ProductDefinitionFieldId
     where field.Name == "Format" && prop.Value == ((!string.IsNullOrEmpty(Param3) && Param3 != "Any") ? Param3 : prop.Value)
     select prop.ProductId).ToList();
 
q4.Add(-1); // Prevents NHibernate from throwing an exception if list is empty
 
var q5 = (from prop in ProductProperty.All()
      join field in ProductDefinitionField.All() on prop.ProductDefinitionFieldId equals field.ProductDefinitionFieldId
      where field.Name == "Presentation" && prop.Value == ((!string.IsNullOrEmpty(Param4) && Param4 != "Any") ? Param4 : prop.Value)
      select prop.ProductId).ToList();
 
q5.Add(-1); // Prevents NHibernate from throwing an exception if list is empty
 
var q6 = from prod in Product.All()
       join desc in ProductDescription.All() on new { pid = prod.ProductId, cc = _currentCultureCode } equals new { pid = desc.ProductId, cc = desc.CultureCode }
       where desc.DisplayName.StartsWith(!string.IsNullOrEmpty(Param1) ? Param1 : desc.DisplayName) &&
       q3.Contains(prod.ProductId) &&
       q4.Contains(prod.ProductId) &&
       q5.Contains(prod.ProductId) && 
       prod.DisplayOnSite == true
       select prod;

Finally got some time to sit down and write a little bit of code. If I'm not mistaken I believe you can achieve the same thing with the following LINQ statement:

var q = from product in Product.All() 
  where product.ProductProperties.Where(property => 
    (property.ProductDefinitionField.Name == "MyProperty" && property.Value == "MyPropertyValue") 
    || (property.ProductDefinitionField.Name == "MyOtherProperty" && property.Value == "MyotherPropertyValue")).Count() == 2
    && product.ParentProductId == null
  select product;

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