Freezing can be caused by all sorts of problems.
1st Try and update the motherboard drivers for your system (you need to know what chipset you have, could be VIA, nVidia or AMD's own).
2nd make sure you have all windows updates
3rd I would probably look at the heatsink you have on that CPU, it may need an upgrade as it could be getting to hot when the CPU has to run flat out.
4th I would look at the rating of the PSU, It may only just cover the systems power needs.
Big system builders will try and cut as many corners as possible when making a system so it could be any reasons.
& yes, video encoding does make your CPU run flat out. This causes alot of heat and the system has to get rid of that heat. Try running with the side of the PC off and a fan blowing into it to allow cool air to enter freely