heatsink under fan of micro choked with dust,pwr supply okey,fans work,drives have pwr,motherboard has pwr but won't load bios.caps appear okey no poptops.
Reset CMOS Resink ram
Run a mem test. If it fails and you have more than 1 stick of ram swap them intill you find the dodgy one.
If these don't work go down to swapping parts.
Swap PSU Swap MB Swap CPU Swap Ram
In what ever order you have these / cost.
bad power supplies can turn on fans but not have correct/enough power to boot windows