I have seen one other post (with an image) of this issue, though not 50 instances of ashWebSv.exe multiple instances. I don't know if that one was also Vista OS, but my XP task manager only reports 1 instance.
I don't know if this would be different if I or you used IE and had multiple windows open ?
I use firefox so only see one window with multiple tabs.
I would normally suggest that you do a clean reinstall, but I don't use Vista nor IE7 so I can't test to see if there would be multiple occurrences of ashwebsv.exe. It certainly wouldn't hurt to try it.
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
Now uninstall (using add remove programs), reboot, run the avast! Uninstall Utility, reboot, install the latest version, reboot.