Daz,

Open up the either the MSconfig window or the Manager in XP. Check to see if the process ashdisp is running.
This controls the icon in the tray.

Do you have “i” icon in the tray?

If you do not see the process, then go into Control Panel, Add/Remove and double click on the Avast line to open up the Options menu. Here you will find an option to Repair Avast. Choose it and then reboot.

See if the icon now appears.