updates prevent rebooting

[ccc508]

installed snow leopard. then did a software update. it installed 7 updates. i then restarted my macbook and it would only go as far as the blue screen after the grey screen with the apple symbol. i also tried to safeboot which took me to the sign in page and relooped back to the sign in each time i signed in. i reinstalled snow leopard and was able get back to my desktop. once again i ran a software update and restarted my macbook. this time it went to the grey screen with the apple symbol and then switched to the circle with diagonal line. i tried to restart again and this time held command s. the script stopped with a line saying something similar to being unable to load mach_kernel. as i am writing this i am reinstalling snow leopard. that's about all i can tell/ remember. i guess my general question is what is happening here? also is there something going on with the software updates that is preventing the macbook to reboot. i guess some info about the macbook: 13 in, white. 4G memory. 250G harddrive. bought sometime between june-august 2008. thanks to anyone with some explanations or solutions.