I am sorry that the info is so vague.
I have a laptop with Windows XP Home SP2 all updates.
Firewall disabled and no other anti virus (or similar) software loaded.
If I instal Avast and re-boot, Windows appears to start normally and the initial Avast (new instal) window pops up, click ok, window closes and then a few seconds later Windows hangs up. The cursor still moves around but turns into an hour glass when pointed at the start menu.
I cannot run any programs.
It is though a background task is waiting to complete.
After several minutes no change.
Ctrl Alt Del does not work.
The only thing to do is power off the computer.
Re-starting in safe mode is a bit hit and miss, several times it will hang up as the startup items scroll past and safe mode will not start.
Eventually after several tries I was able to start in safe mode and uninstall Avast.
The laptop now works entirely normally.
Foolishly I then tried re-installing Avast and had excatly the same symptoms. It took many attempts to re-start before I could get into safe mode and uninstal the software once more.
The laptop is again (without Avast) working normally.
I have used Avast on several other computers without a problem so wonder if it is an issue with this latest version of the software and / or a clash with something else installed on the laptop.
I shall try Norton Antivirus 2006 on the laptop instead, although the Norton software should now be called "bloatware"!
Best wishes to all
Les