The lsass.exe which is from Microsoft is located at c:\windows\System32\lsass.exe . there's a few viruses that have been found to run as lsass.exe to hide from you.
W32.HLLW.Lovgate.C@mm - Symantec Corporation
W32.Mydoom.L@mm - Symantec Corporation
W32.Nimos.Worm - Symantec Corporation
W32.Sasser.E.Worm (Lsasss.exe) - McAfee
Search for this item in the startup DB here.