The windows file system error 2 = The system cannot find the file specified.
What start-up icon ?
avast is a resident scanner and as such should start automatically. The avast desktop icon is normally used to either start avast in safe mode (by default it doesn't) or to run an on-demand scan.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?