This is the older Core2Quad box? Maybe some lower-level hardware diagnostics are in order too. Have you checked the mobo with a flashlight, and made sure that it is nearly dust-free, and checked the capacitors for swelling or leaking? (Many boards around that era had Solid Caps, though, which should still be OK. Most Gigabyte P35/P45/G31/G41 boards did.)
I
might try going into the BIOS, "M.I.T. Menu", and bump up the voltages on the CPU Vcore and Vdimm (DRAM voltage) just a hair. Maybe years of usage have lead to some degradation. Hit CTRL-F1, I think, at the main BIOS menu screen, to "unlock" extra stuff in the MIT menu.
Also try, make a bootable Linux LiveUSB (
www.linuxmint.com ) on another PC and booting off of that, possibly with the HDD or SSD disconnected. That might give you an idea if the problem is your Windows' install, or the storage drive that the OS is on, or whether there's a hardware issue with CPU/mobo/RAM/video.
If that rig has a HDD, sometimes when they go out, you can get "freezes".