Decoding T-SQL CAST in C#/VB.NET

Recently our site has been deluged with the resurgence of the Asprox botnet SQL injection attack. Without going into details, the attack attempts to execute SQL code by encoding the T-SQL commands in an ASCII encoded BINARY string. It looks something like this:

DECLARE%20@S%20NVARCHAR(4000);SET%20@S=CAST(0x44004500...06F007200%20AS%20NVARCHAR(4000));EXEC(@S);--

I was able to decode this in SQL, but I was a little wary of doing this since I didn't know exactly what was happening at the time.

I tried to write a simple decode tool, so I could decode this type of text without even touching SQL  Server. The main part I need to be decoded is:

CAST(0x44004500...06F007200 AS
NVARCHAR(4000))

I've tried all of the following commands with no luck:

txtDecodedText.Text =
    System.Web.HttpUtility.UrlDecode(txtURLText.Text);
txtDecodedText.Text =
    Encoding.ASCII.GetString(Encoding.ASCII.GetBytes(txtURLText.Text));
txtDecodedText.Text =
    Encoding.Unicode.GetString(Encoding.Unicode.GetBytes(txtURLText.Text));
txtDecodedText.Text =
    Encoding.ASCII.GetString(Encoding.Unicode.GetBytes(txtURLText.Text));
txtDecodedText.Text =
    Encoding.Unicode.GetString(Convert.FromBase64String(txtURLText.Text));

What is the proper way to translate this encoding without using SQL Server? Is it possible? I'll take VB.NET code since I'm familiar with that too.

Okay, I'm sure I'm missing something here, so here's where I'm at.

Since my input is a basic string, I started with just a snippet of the encoded portion - 4445434C41 (which translates to DECLA) - and