Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
You may need to reboot after the repair.
Have (or did) you another Anti-Virus installed in this system (before avast), if so what was it and how did you get rid of it ?
Second, since IE is an integral part of your OS you should keep it fully up to date also. I rarely use IE, but I still have IE8 which is the latest version of XP.
If you expand the avastUI Summary section (show details), is it all shields or just some ?
Also check the Real-Time Shield section.
There should be a Fix now option, did you try that ?