Which version of Comodo do you currently have on your computer?
The latest Comodo Internet Security Version 3.12 which has Firewall, Defence+(H.I.P.S) and AV works fine with Windows 7
If you run the installer again(for 3.12) you should get the option to install the Firewall/Defence+ and the AV.
If you don`t fancy running Comodo`s AV you could try Microsoft security essentials
http://www.microsoft.com/Security_Essentials/ which is free or maybe Avast
http://www.avast.com/eng/download-avast-home.html or Avira
http://www.free-av.com/ both have free versions.
Are you used to any particular AV?
Matt