Best and Free tool for Network PC's hardware & software inventory

The easiest to setup and use is as mentioned above, spiceworks. But it has some disadvantages, you need a connection to the spiceworks network, and it isn't OpenSource.

Nagios on the other hand is free and OpenSource, and it doesn't need a connection to a foreign site you have no control of, but it also is much harder to setup and run. This probably the best known and supported OpenSource product:

http://www.nagios.org/

Another OpenSource product similar to Nagios, with the same cons and pros, is OpenNMS. But it isn't as well known and if you don't buy their commercial support and is probably harder to maintain:

http://www.opennms.org/

Then of course there are the products you get with your server hardware, like from HP etc. Those are normally also free, but they can be tricky to setup if you use hardware from other manufacturer's.

@ Eirman
Tried Spiceworks but 80% systems it showed login error and didn't scanned.

@ rindi
Any other?

None I know of.

To my experience, Spiceworks requires tuning to get things just right, especially regarding the authentication portion, but it will do the job.

PDQ Inventory free version works well.  Of course if you need to automate, you must pay. It's cheaper though.  I got my director to sign off on PDQ Inventory for 250 USD.

http://www.adminarsenal.com/pdq-inventory/compare-free-and-pro