Yes of course it can be rewritten in ASP.NET - note that i said rewritten, not just refactored, there is no saving that code (which is okay for what it does, but things are different with ASP.NET).
To be honest, i didn't even check the code, it hurt my eyes. A lot. But in general you can use a nice SQLConnection object, and a SQLCommand, call a stored procedure and get a nice SqlDataReader full of data from which you can build a DataTable or an IEnumerable list of data objects. You then have a repeater type control (ListView, GridView etc) in the UI, simply by binding your datatable or list to that control will render the results.
With the ListView, you specify a template for each data item that is rendered. For a GridView you specify the columns (or column templates) and which properties on each data item the columns should bind to.
When you retrieve the data, you can leave it as a DataTable, or translate it into something else like a list (or array) of data objects. As long as your list/array implements IEnumerable you should be able to just assign the list to the ItemsSource property of the aforementioned repeater control and it will perform its magic.
You don't even have to use a SQLCommand and DataTable object - you could even use Linq to SQL and bind the results straight to your repeater control.
This is just a high level overview of how you could do it, there are a couple of different ways. Once done, your code is going to be way cleaner and more maintainable than the classic ASP code.
Edit: your main issue is how to produce an ordered list, which is what the current code is doing with its nested loops supplying IDs for the detail items.
I would suggest you take a step back, and rethink the sql. In fact, throw the current sql away, it is incredibly inefficient with the tools you have today. It takes literally tenths of a second, and just one database call to return a flat table of data. Your first instinct may be "but i don't want to return too much data!" - relax, even returning several thousand data rows can be a sub 1 second operation if done correctly. You can also restrict the returned data by passing parameters in to the stored proc, or appending them to the dynamic sql statement that you construct (although it pains me hugely to mention dynamic sql, i think it is evil, but some people still use it - i would not recommend it unless it was your only option). To sum up what i am saying, how your data is returned from the database and how it looks on screen are two different things, don't let one guide the other. You can get the data from the database, then manipulate it before rendering it to the UI.
If you still want to show the data as an ordered list, then use a ListView, define a template for each data item, then you can use LINQ to group or filter the data you like (the data item template can contain whatever HTML or ASP.NET controls you like, and that template gets rendered for each data item in the list of data). Alternatively, you could use a GridView, and then use the grouping capability of the GridView to do the work for you - just specify which column(s) you want to group on, and the GridView takes care of the rendering.