wonder if someone could help me please

my computer has been running slow and freezing for a while, but its not been a major issue, but last night my antivirus alerted me to an infection and before I could do anything the computer rebooted itself....but then the antivirus (which was avast) is not working, tried to run spybot wouldn't let me open it so I downloaded Explainer video to try that and the same thing.tried uninstalling and re installing but nothing
I unistalled and re installed Avast and after 3 attempt's it worked for about an hour and again it is saying it is stopped or in an inconsistent state and will not let me fix it
I tried running a scan via avast but it seems to be stuck at 97% and I have no clue what to try next
any help would be much appreciated.