vista wont shut down.

hey all: I am running vista home preimium and it will not shut down. I installed sp II today and still the same problem. I sometimes get windows explorer error messages. I am running 1gig of ram and have ordered another gig but i dont think that is the problem. Every once in a while it will shut down correctly. I dont know if cleaning my registry would help and if it would is there a free program i could try. Thanks for any help.......i am just a novice learning the ropes but am a fast learner.

Could be an application causing the problem.
Have you tried "shutdown -f" in run? See if that works and let us know.