Unknown PCs with Scan:
As a new user to Spiceworks it might be good value to you to have a look at some of the training videos to get a quick grip of some of the basics, they only run for 1-7 minutes and can really get you going: http://www.spiceworks.com/tv/?cat=training
Then there are also lots of Help and How-tos that will help with some of the details.
Resolving Antivirus and Firewall issues in a Domain: http://community.spiceworks.com/help/Configuring_AV_Firewall_Domain or Workgroup: http://community.spiceworks.com/help/Configuring_AV_Firewall_Workgroup
DNS issues: http://community.spiceworks.com/help/Resolve_Your_DNS_Issues
Testing Authentication: http://community.spiceworks.com/help/Using_Spiceworks_To_Test
Troubleshooting WMI: http://community.spiceworks.com/help/Troubleshooting_WMI
Getting scan to work with Vista (and Windows 7 UAC): http://community.spiceworks.com/how_to/show/317
Resolving Spiceworks Unknowns Script to make changes to the PCs with ease: http://community.spiceworks.com/scripts/show/181-resolving-spiceworks-unknowns-unofficial
Enjoy… 
Authentication errors;
Check WMI settings: Setting The Default WMI Namespace Security or Troubleshooting WMI
Maybe you want to run this script at the affected machine (works in all of my cases): Resolving Spiceworks Unknowns Unofficial - Script Center …
If this doesn`t help check this for authentification issues:
and this for Firewall and AV issues:
From bytesnake:
Use the 7th or 10th link (it`s the same) of Air Jimi (Resolving Spiceworks Unknowns Unofficial - Script Center …) and comment out the lines 36 - 38 to keep UAC. Then follow the 6th link of Air Jimi (http://community.spiceworks.com/how_to/show/317).
If you can live with disabled UAC, just run the script (with all lines active).
I suggest to follow this: Troubleshooting Spiceworks Inventory Inconsistencies - Spiceworks … and set scan speed to slow additionally.
