first, look in your eventlog, start->run-> eventvwr
look under system and applciation for a winlogon event. Look through them for finding the amount of problems checkdisk found. They should tell you everything you need to know about your disk. It's possible that you are having a problem with sectors, but we won't know unless you look. Also look for disk errors. This will indicate a problem.
also, read this site. There is a ton of info on it and looks like it might be helpful to you...
http://www.alegsa.com.ar/Visitas/i64...rag%20runs.php