You may have a piece of malware that has disabled them to make life difficult to remove any malware on your system.
So the reason you can't open batch files or the cmd.exe isn't avast related as it doesn't block, but scans and alerts to infection.
avast has a self protection module (Program Settings, Troubleshooting) that will stop you disabling avast processes, etc. You can pause the Standard Shield stopping any on-access scan or you could Stop On-Access Protection (all resident scanners) right click the avast 'a' icon, select Stop On-Access Protection (these you will find makes no difference as it isn't avast blocking).
Having two resident scanners installed is not recommended as rather than provide twice the protection it can cause conflicts that could leave you more vulnerable.
If you haven't already got this software (freeware), download, install, update and run it and report the findings (it should product a log file).