vista reboot loop after adding new ram

I believe putting the second stick in, buffled the memory controller. What i would do in your case is:
1) Increase Vmem voltage (the voltage that goes to your memory)
2) Increase the Vnb voltage (voltage that goes to northbridge) - Northbridge is the chip that usually contains the memory controller.
3) Increase the memory timings a bit (eg. from 4-4-4-12 to 5-5-5-15)
4) Change the memory strap (eg. from 333MHz to 266MHz or vice versa)

Good luck!

thanks for responding
but my problem is that the bios is very simple and doesnt allow me to do all this things ( i could only change the memory strap speed ) and there is no bios upgrade for my mobo.

my problem that the computer doest accept more than 512 mb ram
adding any more ram will cause windows not to open or some times when it opens it becomes unstable and compter could restart at any time

also it is not os problem cause it happened also in xp.

Sorry mate, but if your BIOS does not allow you to change basic stuff like memory voltage, then the only option left would be to find a hardware mod (mod short for modification) to increase the voltages of the memory and the Northbridge.

In my opinion, first of all a board that doesn't allow you to change any of those settings is not worthy at all. See for your self, buying extra RAM and you can't use it because you can't access any of those settings in the BIOS. That is ridiculous. Anyway, your best bet would be to email your motherboard manufacturer and tell them your problem. Maybe they have a workaround. Otherwise... you are stuck with the extra RAM and a crappy motherboard.

PS: Did you check your manual if your motherboard actually supports the memory you bought? Some mobos don't support double sided memory sticks etc... it may be one of those weird mobos. Your manual should state that. What kind of memory modules it accepts and in what places (DIMM slots) and what configurations.