Can't delete Avast Antivirus

I had troubles with my Avast Antivirus program and wanted to delete it but always says I don’t have permission. No other software I tried to delete it either. My system is Satellite A215-S4747 with Win Vista. I tried reinstalling but has permission issues. What to do???

You don’t say what Avast Program and crucially what version number you are using (given you are running Win Vista) ?.
Though you have posted in the Avast Cleanup sub-forum, which is an avast product rather than, how to clean-up and existing Avast Antivirus program.

There is a method but I don’t know if it would be compatible with what could be a very old version of Avast for that Operating System.
Clean Reinstall