Firstly it wouldn't matter when you installed 4.7, so long as you have kept it up to date.
avast does support XP64 and Vista64 for that matter.
The fact that your icon is missing is not because you have XP64 and the most common cause is another security application with an over aggressive start-up protection which could kill the ashDisp.exe (icon and interface to the on-access providers, etc.).
I don't know how these suggestions below differ, if at all in XP64, but I'm sure you can find the equivalent in XP64.
avast! icon missing - As a temporary measure until this is resolved you can create a desktop shortcut for this file C:\Program Files\Alwil Software\Avast4\ashDisp.exe (the avast icon and interface to the providers). Right click on the file and select Send To, Desktop (create shortcut). You will need to run this after each boot until the problem is resolved.
What other security based software do you have that might block new startup entries, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, Spyware Doctor (StartUpGuard or OnGuard), PrevX, WinPatrol, ProcessGuard, etc. ?
Some of thes applications won't have a 64bit version, but they are really only to jog your memory as to what security applications you may have.
Check the option in the Appearance tab of Program Settings. Or Make a link to ashdisp.exe in your startup folder.
Or Add the path to ashDisp.exe into a value named avast! in the Windows Registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run (see c:\avaststartup.reg if help needed)