Go into the bios PC Health screen and see what the reported voltages are.
Make sure that all displayed values are correct.
Do you have a friend who has some basic electrical knowledge.
If yes, see if he has a DMM (Digital Multi-Memter) and he can check voltages at several places on the psu connectors.
You can also verify if other software monitoring programs display similar values.
Which version of HWmonitor are you using?
EasyTune6 might show 12 volt readings in one of its panels.
The attached diagram shows standardized psu connectors and the voltages associated with each wire.
**edit** Make sure that all the psu connectors are fully seated, especially the 24-pin and 8-pin connectors.
With your system turned off AND your psu switched off, remove and re-seat all psu connectors.