Problems with my computer

Doesnt sound like a short circuit, they tend to just outright prevent startup.
HDD may be failing, run a Hard Drive health check program (cant think of any off the top of my head, sorry).
RAM may also be the issue, run Memtest to clear that as the problem.
Doesnt sound like a PSU problem, that would normally just shut-down the machine instead of crashing it.
Run a Virus sweep in safe mode as well, unlikely to be the problem, but who knows.
Have you overclocked the CPU at all? If you have it may be an unstable overclock (seems like you have found the tipping point of instability if you have)

http://www.memtest.org/
EDIT: A Hard Drive health and performance checker!
http://www.hdtune.com/
You can also check HDD health through Windows as well. Open "Computer", right click the drive in question, Properties, tools, use the error checking tool.

Thanks, i also did a memtest and they weren't the problem forgot to list it above but i'll try the HDD test, thanks

i did a hard drive test and no problems were found

do u get a blue screen at al ? if so check out a free program called who crashed me, it is a really good clean app that i use. whenever i play a mmo i randomly get crashes with a bsod and that app was able to tell me that my cpu clock wasnt responding in the alloted time or whatever its called lol

start with the simple stuff. in the bios make sure the efi speed is set to standard not performace. setting it to performace will change the cpu and ram bus timing from 1:1 to 1:5 and can cause issues. make sure in the bios you have the dram speed set to xmp profile. in the bios check that the sata ports are set to achi not raid or ide mode. for your cd-rom drive and hard drive check that there on the intel sata ports. if your mb has only one 6g port leave that open for a ssd. a hard drive will not max out a sata 3g port yet. check with the mb install guild that the ram is in the right slots and is locked into place. also check that the ram is on the mb qal list. most new ib ram is going to be rated at 1.5v. at 1600 speed. if you have faster then 1600 speed ram use cpu-z and under the spd tab will be the rated speed/voltage/timing for your ram. you may have to set it up Manuel in the bios. also try running prime 95 blend on. and have hardware monitor running logging your temps and voltages. it may be a over heating chipset or a cheap power supply or a power supply that has an issue holding one of the power legs to the correct atx voltage.

I probably should of mentioned that when it crashes, the computer is still on, i just have no video output, apart from sometimes when there is black lines. I then have to hold down the power button to get it off and when i switch it back on it says that windows was shut down incorrectly.

any more suggestions :S?

Try running the machine bread-boarded. Instructions on how to do so are halfway down the first post (may also be worth following the whole guide). Except connect a monitor to it so you can see what your doing in Windows.
http://www.tomshardware.com/forum/261145-31-perform-ste...

Remove all hardware that isnt required for it to boot (extra HDD's, graphics cards, optical, all but 1 RAM stick, etc). Boot the machine back up and run Prime 95 (you say that it crashes when doing intensive activities, this is as intensive as you can get). If the machine runs fine, start adding components back in until it fails again. If it fails initially, replace the RAM and try again. If it keeps failing at the start, you can be sure its either the mobo, CPU, PSU or HDD causing the issue. If you have Linux on a USB or something try that as well, it will eliminate the HDD as a suspect if it fails there as well. If you have another PSU, swap that in to also eliminate it as a suspect.

This should test for defective hardware.

EDIT
Oh yeah, to turn the machine on while in this state and not connected to any power buttons, all you have to do is use a screw driver to short-circuit the two pins that connect to the front power button. The board may have an onboard power button.