I have two computer, all use the free home version.One alway goes crush after intalling the version when restart.when it happens, I can do nothing but move the mouse point, numlock is dead.If I hold “shift” when restart, the crush won’t happen.I uninstall the avast, everything is ok. I use other antivirus software, no crush.
For another computer,the crush happens only sometimes, the symptom is some when crush.
I am in China, and I use the winxp sp2.In the internet, I have searched the same problem, to my surprise, a lot of people get into the same truble. I believe avast 5.1 can conflit with some function of the system, or other software.
Hope it can be solved soon, because I love it.
Hello Jeff.xiao: I just want to point out, that having other Anti-Virus programs installed on your machine and not uninstalled properly (with venders uninstall/Clean-up tools) can leave remnants (left overs) that could (and often does)cause problems with the installation and proper operation of Avast. :-\
Apart from the likely confliction with another installed antivirus (mentioned above) your Windows version is very out of date and no longer supported by MS. SP3 (+subsequent updates) is the current for XP.
I don’t know if that might be causing this problem.
Thank you for your help!
I have to clarify that I haven’t installed other antivirus software before,I just tried other software after the crush. so I can make sure that the crush is not because of the conflict between avast and other antivirus software, that is to say, avast may conflict with some microsoft system functions or sound driver,etc.
xp sp2 maybe is the reason, but I think it not so out of date, for an exellent software, the crush should be avoid at any case.anyway, other competitor sw does it well, I hope the defect can be digged out.
I report exactly the same issue with the latest avast update.
Computer hangs on the login screen, keyboard numlock not responding, only mouse can be moved.
I do have the latest updates (win xp pro).
I had to uninstall avast (in safe mode). Only then I could boot in windows.
The same happened to my other win xp machine.
Unfortunately, it seems that you don’t acknowledge that this is a problem, so i expect no fix is coming soon. 
Please read:
Support for Windows XP Service Pack 2 ends on July 13, 2010
http://support.microsoft.com/gp/lifean31
XP SP3 has many Critical Updates and several performance enhancements.
@andru123,
What is the CPU and RAM of the affected machines?
Any other security software installed?
You could try a full uninstall and re-install, using the Avast uninstall utility After the initial uninstall. Reboot after uninstalling, run the utility, reboot again, then install the latest version. Reboot as directed after it’s installed. If you are low on RAM, don’t rush it.
andru123,
seems we have the same problem,my two computers have 1G/2G RAM, cpu also powerful.a lot of people on internet also report the same problem. I still prefer avast 4.8,stable and secure. after I update to 5.1, computer become unstable, the desktop crush at starting,(I don’t need to use the safe mode, I just hold the shift key when start, so I’m sure there is conflict with the loading programs),my laptop sometimes crush at starting, and blue screen when waking up yesterday(ftdisk error).
I have the same programe :‘( :’( 
I have a similar problem on an asus eee pc 901 running win xp sp 3 and zone alarm. Avast free v 5.0.667 works fine but on installing 5.0.889 on reboot the computer starts loading XP displaying the logo and g :-[reen file load animation then locks up with a blank screen, a reboot into safe mode and uninstall and re-install v 5.0.667 fixes the problem.
Robert
I have the problem too. I’m running Windows XP SP3 and I use ZoneAlarm Pro. I cannot run any programs after bootup (including task manager). I resolve by booting into safe mode with networking, uninstall Avast, download a fresh copy and install.
Boot normally, I can now start programs.