Use Windows System Assessment Tool (WinSAT) to obtain a Windows Experience Index (WEI) rating.
The benchmark you use will depend on what you want to benchmark. You may use some of the freewares available on the net.
NovaBench and Sandra focus on general performance, such as processor and hard drive speed. HyperPi for your CPU only.
FRAPS and 3D Mark are the ones to use if you’re interested in gaming performance.
EDIT: To check stability, run prime95/Intel burn test for a few hours. GPU: Furmark; RAM: Memtest86+.