I have the same problem

If you are using windows xp, then follow these steps:
1) right click "my computer" icon, select "advanced" tab, click settings button towards the right of "start up and recovery" and  click "edit" tab
a notepad window appears with some text in it which reads something like this :

[boot loader]
timeout=0
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /FASTDETECT 

modify that text to look like this:
[boot loader]
timeout=0
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /FASTDETECT /3GB /userva=2800 /NoExecute=Op

Warning : only the "/3GB /userva=2800 /NoExecute=Op" has to be added extra else you will mess up

2) save the notepad and insert max ram, and check, if any BSOD's
if the problem persists, then it is hardware related..
Most probably one of your ram stick may be the problem child(which may be either incompatible with your motherboard or defective)

Best of luck...