How long did you wait for the startup? I usually go for a little walk or stretch myself a bit if it takes longer, but it has never failed to boot for me since the last two years at least.
Personally, I can’t remember having problems at startup with Avast and I’ve just switched from CF 5 to 6 on my W7SP1x64 laptop in the last couple of weeks.

I’d suggest the following:

  • be patient when it happens or get a SSD if you don’t have one
  • try clean install of both Avast and Comodo
  • check S.M.A.R.T with CrystalDiskInfo and post a screen shot showing every raw value meaning expanding the window
  • if it comes to the worst switch either AV or FW or try a complete clean install of Windows and everything

Also, listing your computer specs like

  • CPU
  • MB
  • RAM
  • GPU (especially with the GPU driver version)
  • PSU
  • HDD/SSD

might help in debugging the issue.