Without a little more detail on your hardware, it is difficult to suggest which component may be causing this. Which memory are you using?(make and model),
Have you tested the issue without the external GFX card (just use the onboard) and also what is the make and model of your power supply?
Sounds like it could be either memory, psu or possibly hard disk. Are there any entries in the Windows Event Log following the restart, does it happen in safe mode?
thx for your answer, the power supply that im using is the power up 550 w atx, the mory i used 2 kind first i was using 2 memory of 1 Gb each the brand was nvidia, the part number is ocz2n800sr2gk and it work at 800 Mhz, then i used centon, 2 memory of 1 Gb each they also work at 800 Mhz and the part number is 2GBDDR2KIT800 and it was worst because with those memory the computer reboot after a couple of sec, i checked Windows Event Log and it say your computer shut down without reason, click the potion that allow me to check online and when i go to the microsoft website they dont have any error for this, i hope this information can help you to help me, thx again