Hi,
Avast keeps telling me that my virus file is not up to date. When I click on the link in the box that appears above the Sys tray, it logs on to the server then comes up with the following message telling me I am already up to date. It does this every single time I log on and is getting a bit stale now.
Information about current update:
Total time: 10 s
Server: www.iavs.cz (217.31.49.244)
Downloaded files: 3 (0.03 KB)
Download time: 7 s
Are your system date and time correct?
Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair). You must be connected to the internet while repairing.
Are you sure it is avast "Avast keeps telling me that my virus file is not up to date. " and not windows security center?
Where is this displayed and when?
Check your system date, especially the month and year.
Silly me. replaced mainboard a couple of weeks ago and never set date
Welcome to the forums, Neil ! 
Sometimes, the problem is so simple that we overlook the solution. 
Be sure to come back often, learn more, and maybe help others. 
Glad you have it sorted now, welcome to the forums.