This problem arises because the default upload limit of IIS is set low, however you can increase the upload/download limits by changing some settings:
For IIS 6,
There is a file named metabase.xml in C:\windows\sytem32\inetserv which keeps some settings of IIS 6.
Before editing this file, you will need to either
1. Stop IIS and restart it after you save the file.
Go to Start > Run and type CMD and enter the following:
To Stop:
%windir%\system32\iisreset.exe /stop
To Start:
%windir%\system32\iisreset.exe /start
2. Check the box that allows metabase editing on IIS Manager and edit the file on the fly without having to stop or restart any of the IIS services.
You should edit this file carefully beause IIS will not work if something is missing in the file. You should better make a backup copy of this file. Open this file with notepad or some other text editor. Find the setting named "AspMaxRequestEntityAllowed".
For example the current setting may look like AspMaxRequestEntityAllowed="204800"
This means you are allowed to upload upto 200KB (204800 bytes) of files. So for instance if you would like an upload limit of 20MB then you can set the value to 20971520 (or to 20000000, it doesn't have to be a precise value). You can also change the setting named "AspBufferingLimit" if you experience any problems with downloading of files, the same rules applies to this variable also but for download instead. However if you are using v3.00.b23 you shouldn't experience download problems because it's optimized for IIS6. Save the file after your changes and restart IIS as stated above if you are not editing on the fly.
For IIS 5,
You will need to edit a registry setting instead, please see this article:
Description of the MaxClientRequestBuffer Registry Value