In the Nessus FAQ it says
Microsoft Windows desktop systems have network limitations that may impact the performance of Nessus. The TCP/IP stack limits the number of simultaneous incomplete outbound TCP connection attempts. After the limit is reached, subsequent connection attempts are put in a queue and will be resolved at a fixed rate (10 per second). If too many enter the queue, they may be dropped.
And the recommended settings there does not the options in Nessus 5 and I am confused about what to set in the product.
I wonder what settings you use on a Windows 7-desktop machine to make sure the scan is as reliable as possible?