the ram could be faulty or your laptop can only take a max of 2gb. vista 32bit can support up to ~3.5gb, where as the 64bit version supports up to 128gb. this wouldn't cause the blue screen though. i would just try each stick separately to see if they both work, if they do then i would think your laptop does not support 4gb ram.

if you turn automatic restart off by going to system -> advance -> error reporting , you can see what the blue screen is actually telling you.