HWiNFO is a popular and detailed system monitoring software. It recently turned commercial but the developer was kind enough to allow an integration with Rainmeter when used free for personal use.
What basically happens now is that HWiNFO is reading the system measures and puts them into the Windows Registry. From there, Rainmeter skins can pick them. In fact, the integration has now become much easier.
Read all about how to install and set up HWiNFO here.
Edit your HWiNFO.inc File
With these information at hand, you need to edit the HWiNFO.inc file in the @Resources folder.