The front headphone sound comes from the same chip on the motherboard that supplies the sound to the rear speaker output.
When you reinstalled, did you properly reinstall everything? OS, motherboard chipset drivers
, device drivers including the HP supplied sound driver
? Windows 7 will install a lot of drivers, some are the correct drivers and some are not, thus you need to install the PC vendor's drivers for some devices, especially the sound.
As you were inside the PC, accidental damage to the motherboard is a possibility due to the motherboard having static sensitive devices on it. Check the internal cabling and especially the cable that connects the front panel audio jacks to the motherboard, to make sure its properly seated. To see if the front panel connection is causing the low output to the speaker connection, temporarily disconnect the front panel audio connection (check or remove the front panel connection with the PC powered off. If you don't have a grounding wrist strap, at least ground yourself to a metal part of the chassis before touching anything inside).