cltui.loc file infected? Meaning?

NovaBackup (my backup program) has started reporting

WARNING: Backup: File infected: C:\Program Files\Norton Internet Security\MUI\18.5.0.125\09\01\cltui.loc: File Skipped

 

What does this mean? I never used to get any warning message when doing backups before. 

 

I'm running Norton Internet Security (how do I find the version number - I don't see a Help About button)

Windows XP Pro SP 3

2.5 Gig memory. lots of free disk space.

Thanks.

Mark