Why does the lsass.exe process use so much CPU?

I'm using XP SP3 on my computer at home. A few weeks ago I noticed my
computer was using a significant amount of CPU even though no applications
were running. Using task manager I discovered that lsass.exe was using 30 % -
50% of my CPU constantly. I have never seen this behavior before. Overall
performance on my computer has been abysmal since then.

Is there any valid reason why this process is using so much CPU?

I've browsed the web and found lots of articles on viruses using lsass.exe
as as vector, but nothing the seems to be helpful. Using Process Explorer I
tried to validate the process, and Process Explore thinks it's a valid
Microsoft process.

I've been in touch with Bit Defender, my security software provider, but
they have not really been of use.

I'm still convinced there is something not quite right with my system, but
the virus scans don't turn up anything. I have notice the for some reason the
latest Windows updates seem to fail. My disk keeps getting full even though I
am not adding any files, and routinely cleaning out temporary and cache files.

Any help or thoughts would be greatly appreciated.

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Eric Kolotyluk
Software Developer