Audit failure on tcpip.sys - Jef

I have Vista Ultimate 32bit.
I have all the updates including SP1 that I got from Microsoft.
Looking at my events log I see

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\drivers\tcpip.sys

I dont have any patches or anything not from Microsoft that I know about.
Is this just a bug or something to worry about

I use windows firewall only and NOD32 for antivirus which comes up clean on
a full scan
Thanks for any info
Jeff