Hi,
"avastantivirus.exe" is not from Avast! installation -- you can delete it with no fears.
Task manager can be disabled in registry: "HKLM(HKCU)\Software\Microsoft\Windows\CurrentVersion\Policies\System\DisableTaskMgr", Value=1
so set value to 0.
if the regedit tool is also disabled "HKLM(HKCU)\Software\Microsoft\Windows\CurrentVersion\Policies\System\DisableRegistryTools", Value=1, solution can be found here:
http://www.pcreview.co.uk/forums/thread-1713099.phpAlso check (and delete) keys in
"HKLM(HKCU)\Software\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\" subkeys with key "Debugger" set to i.e. "ntsd -d" (except key "Your Image File Name Here without a path").
EDIT:
if you can't run regedit.exe after you sets "HKLM(HKCU)\Software\Microsoft\Windows\CurrentVersion\Policies\System\DisableRegistryTools" to value 0, then it could be caused by "HKLM(HKCU)\Software\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\regedit.exe", so you can rename/copy regedit.exe to another name i.e. re.exe and try to run this one.