lsass bootup problem

When I start up I get box and in the header is "lsass.exe - Unble To Locate
Component. Inside the box is "This application has failed to start because
LSASRV.dll was not found. Re-installing the application may fix this problem"


If I click OK or close the box nothing happens. I get a black screen with a
cursor tat moves and the three ifnger salute does nothing. I can't start in
safe mode - I get the same thing, nor will last known good work. I renamed
the old lsass and LSASRV file and extracted new copies from the disc but that
didn't fix nor did running chkdsk /r.

It sounds like the old Sasser virus or a variant thereof but I'm running
Service pack 2, all updates are current and I have AVG and MS Anti-spyware
running too.

I see an earlier mention of this problem but no resolution, is there a fix
or do I have to run a repair install or even a new install?
--
computers and software keep me humble