Logged in over remote desktop, I just saw this error. Obviously it's slightly concerning to see that an "unauthorized change was made to Windows" since this error message showed up while the computer was in a screensaver.
Some event log digging yielded this:
Code integrity determined that the image hash of a file is not valid. The file could be corrupt due to unauthorized modification or the invalid hash could indicate a potential disk device error.
File Name: \Device\HarddiskVolume1\Windows\System32\l3codeca.acm
Running Vista Ultimate x64. Can someone MD5 that file and tell me what they get?
My results:
c:\Windows\System32>md5sum l3codeca.acm
3bb9e6aec59f77f5ea935591bef18c5e *l3codeca.acm
c:\Windows\System32>sha1sum l3codeca.acm
59c873741391cb4abf7783b3de119f11a42192c0 *l3codeca.acm
I haven't installed anything in weeks either. Slightly annoyed and concerned. 
I initially couldn't access the control panel either. I rebooted and didn't see any follow-up errors and the control panel worked again.
I would be very appreciative if someone could shed some light on what's happening. I see that some programs apparently trigger this, but I don't have any of the listed culprits.