Local Security Authentication Server (LSASS.EXE). This is the LSA server. During user authentication, the WINLOGIN process will interact with the LSASS process. LSASS implements the user space part of the authentication procedure for accessing objects, interacting with the Executive Security Reference Monitor mechanism.
If you're having problems with LSASS.EXE then please read the informations about the worm "W32/Sasser":
*vil.nai.com/vil/content/v_125007.htm
In this context you could find avserve.exe on your system