VISTA lsass.exe I/O's constantly. not worm

I've already checked for the lsass worm. this is the 'real' lsass on a 32-bit Vista, IBM Thinkpad T61. This is the last issue I have - otherwise its running as well as Vista is capable of to my limited knowledge. But the lsass process seems to cause the disc to be slammed on every reboot/login, and the drive will keep thrashing for about 6.5 minutes, then finally quiet down. but even with nothing at all running on the system, lsass will continue to hit I/O Writes and Reads all day long, at a much slower pace of course. The total I/O's is more than 10x what I see for lsass on my constantly-used notebook running Win 7 Home Premium.

should I just copy over a fresh one and give it a try or is this "normal" for troubled vista? Is there a way to hack it so that once its job of authenticating is done it unloads itself?