Browse to the Windows System Temp folder and give the ASP.NET account write, read, and read and execute security permissions to this folder. This folder is usually located at “C:\%Windir%\Temp”.  

However, if you have a custom setup for this folder location, the folder can be viewed by right-clicking on My Computer and selecting Properties. Once into the Properties window, click on the Advanced tab and then click the Environment Variables button. In the lower box is the entry for the systems Temp Folder and its location.

Verify all TEMP folder permissions to assure that ASPNET, and IUSR_server name account have read, read and execute and write permissions in all TEMP folder locations.  Some places to look:
a) %tmpdir%
b) %tempdir%
c) C:\Documents and Settings[Computer Name]\ASPNET\Local Settings\Temp folder
d) C:\winnt\temp