avast from windows boot?

[guink666]

I have a virus that seems to disable avast. Ive tried updating it and still nothing. I believe it is still installed and running, but if I try to open avast I get the windows hourglass for a second and that's it. I also can't bring up the windows task manager (ctrl,alt.del). Since I can't open avast I can't run a virus scan. Ive even tried in safe mode. I'm ready to just do a system recovery, but I'd rather not (if I can help it) I'm not a computer wiz so I hope I've explained this o.k.
So my question: Is there a way to do a virus scan from windows boot menu? Windows XP

[Scan]

When you install Avast you get to boot scan so yeah
but still Download malware Bytes and Scan and Remove every thing it finds

http://www.malwarebytes.org/

[DavidR]

@ quink666
So when you open the task manager what avast processes are running, they begin with ash or asw, see image ?

How are you trying to open avast ?

I take it that the avast 'a' tray icon isn't there ?