Are you getting any errors on boot about no protection or other messages, if so what ?
Have you made any changes to your avast installation ?
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
Open the On-Access Protection Control (right click the avast icon), try starting the processes, does that stick when you reboot ?
Check that the avast services (ashServ.exe and ashUpdSv.exe) are set to auto, sorry I can't say how to check that I don't use Vista. Windows Start, Run, type services.msc does it in XP.