Lsass.exe memory leak on windows 2003 server

In the past month or so I noticed that lsass.exe has started to leak memory, getting to 500MB+ of ram in under a week after reboot. Before this I had never noticed it using any significant amount of memory compared to other processes on the system.

This is happening on 2 identical servers, neither of which has anything to do with Active Directory.

Maybe a recent Windows Update has caused this? Any thoughts on things to check?

As a side question is there some way to recycle the memory usage of lsass.exe without rebooting?

Edit:

Here is what I'm seeing in Process Monitor, there are thousands of registry open/query/close a minute from lsass.exe. How can I track down what is triggering these?