Can anyone help please? i've searched the net and can't find an answer.
I have a:
Biostar p4m900-m7fe motherboard
Intel p4 cpu
450w psu
2x1gb kingston ddr2-667 memory
HD3450 256mb graphics card.

With this combination vista won't boot, the mobo beeps once but then goes on a loop with no start.
If i use the onboard graphics instead of HD3450 it works fine, and if i use all as above but with a 1gb ddr2-667 and a 256mb ddr2-400 instead, it also works fine.
I have also tried a 128mb ati x700pro graphics card but to no avail.
I can't fathom this out at all and presume there is a simple explaination or solution for this.
Any help would be appreciated.

Its maybe because you didnt uninstalled drivers from previous graphic card. you cant have diffrent drivers than graphic card.

The above post is correct, however you also need to disable the onboard video chipset altogether if you wish to use a dedicated graphics adapter. This can be done in the BIOS when you first start the computer, usually by pressing the Del key while the post information is being displayed. Once in BIOS, you need to locate the settings for the onboard video. There should be an option to disable the device. Only do this after the drivers have been uninstalled. You can also run Driver Sweeper to clean up the driver installation. Once these two things have been done, you can install the new graphics card and install the new drivers.

I have uninstalled it and disabled it but still no boot. What i can't understand is that i can get the HD3450 running but only if i have 1.25gb memory in. Everything is working just not in the combination i want. I have built many computers and this has never happened before.
Thank you for any advice given

i have just tried a fresh install of vista, the install runs fine with 2x1gb but when it needs to restart the system, the same thing happens, it just won't start windows.

maybe your motheboard doesnt support that ram, i dont know...