Of the free tools available, this is by far the simplest to use if you can get yourself a copy. Tenable security have/ will discontinued support for NeWT, but they have a new product coming out to replace it. The upcoming version of Nessus 3 for Windows will replace NeWT and will not have any IP-based scanning limitations. Tenable expects to release Nessus 3 for the Windows platforms shortly, it can be downloaded from here - www.tenablesecurity.com/newt.html
Languard Network Security Scanner www.gfi.com/lannetscan is another good free package. After installing NeWT, go to Start-Programs-Tenable Network Security-Tenable NeWT-NeWT Security Scanner. This will check through more than 4000 common security vulnerabilities. To do this, click New Scan Task.
The free version can only check a local network and, for standalone users, you will see the name Localhost listed on the drop-down menu. Select this and click Next, then choose "Enable all but dangerous plugins" before starting your scan. Note that the report generated by NeWT will only be useful if you have advanced knowledge of ports and network connections.