Hmm, if you were not using a share when your problem occured, your system itself is having some problems. I can not really think of any likely causes for this other than perhaps a hard disk that is about to die? Could this be the case? Or perhaps you are just stressing the system very hard (e.g., running another appplication or virus scanner that eats away a lot of resources).
Hmm, I've upgraded quite a few systems to SP3, and the only time I had problems was when upgrading a system with an AMD processor (Microsoft goofed up and accidentally installed Intel drivers on such a system). This problem has been fixed a long time ago though and everything should go smoothly now (as a precaution, always take a backup before updating though).
Even if it is not related to your problem per se, updating to SP3 is still really advisable, even if alone for the security measures that have been incorporated into it.
Cheers