I will extend my answer to your question to take into account other possible situations.
A very good link to learn about uploading big size files is this one:
http://weblogs.asp.net/jgalloway/archive/2008/01/08/large-file-uploads-in-asp-net.aspx
Here Jon Galloway explains the best techniques to treat the problem:
1.-Changing machine config or web.config:
<system.web>
<httpRuntime executionTimeout="240" maxRequestLength="20480" />
</system.web>
Here you change not only the maxRequestLength, but you must give more seconds for the executionTimeout.
Interesting thing: Take into account that the value of this setting is ignored in debug mode. The default in .NET Framework 2.0 is 110 seconds. In the .NET Framework 1.0 and 1.1, the default is 90 seconds.
2.-Talking about the real solution, HttpModules like the free of charge NeatUpload
3.-Explaining another way of uploading more intuitively: Silverlight or flash swfupload
4.-He speaks about one restriction II7 has. In this page http://www.telerik.com/help/aspnet-ajax/upload_uploadinglargefiles.html you can find more interesting settings for IIS 7, to set a maximum of 100 megas.
You add:
<system.webServer>
...
<security >
<requestFiltering>
<requestLimits maxAllowedContentLength="1024000000" />
</requestFiltering>
</security>
</system.webServer>
And you must open the file C:WindowsSystem32inetsrvconfigapplicationHost.config and find the line:
<section name="requestFiltering" overrideModeDefault="Deny" />
changing to:
<section name="requestFiltering" overrideModeDefault="Allow" />
Another interesting thing Galloway mentions: "In ASP.NET 1.0 and 1.1, the entire file was loaded into memory before being written to disk. There were improvements in ASP.NET 2.0 to stream the file to disk during the upload process."
For IIS6 the solution Chris gives I think is appropriate:
http://www.banmanpro.com/support2/File_Upload_limits.asp
Another source:
http://www.telerik.com/support/kb/aspnet-ajax/upload/page-not-found-error-when-uploading-large-files-on-win2003.aspx
Another URL where one user has tested a lot of components here:
http://remy.supertext.ch/2008/01/file-upload-with-aspnet/
He refers to a codeproject project (!) that is another very good example of using large files and flash here:
http://www.codeproject.com/KB/aspnet/FlashUpload.aspx