Last time I had this it was a faulty hard disk try booting from the install disk into recovery console and running ChkDsk /R on the boot and system partitions (if different). If that gets you going again then clone the disk and put the new disk in it's place. You should always have disk mirroring/duplexing on essential servers. LSASS is the local security administration sub-system.
If you have just been changing security settings then take a look at http://www.jsiinc.com/SUBF/Tip2500/rh2599.htm