Hello there. I have installed Avast! Home edition 4.5.549 on a P IV based system with Windows XP w. Service Pack 1 & 256 MB of RAM and right now Im having an unusual problem - the program is, obviously, running, both of the small revolving icons, for VRDB and Resident Protection are noticable and running, and I can access the menus with the right mouse click on them, but I cant start the Antivirus or the Resident Protection window; I can access the updating option and can stop the resident Protection from running, but once I do that I cant start it again... I also cant start avast! Antivirus via its desktop icon or the Start meny... since Ive installed it properly and with default options, im clueless why it isnt working ??? please help.
Try a repair of Avast.
Milan, I suggest you run VRDB manually and wait it finishes…
After that, you can repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the popup window (Repair).
If you can’t restart the providers, you could have an RPC error that is, generally, corrected by ‘repair’ function 
Ive tried the repair and it all worked fine... took only several seconds; sadly, after the restart, nothing changed - double click on the small icon in system tray, or the one on desktop still doesent open programs - however, it still looks as if program IS working…
Did you install Avast when logged in as administrator (XP Pro) or as user with admin rights (XP Home)?
I have installed it as a user with admin rights (its a family PC and I havent set up any admin password)…
But anyway, Ive de-installed 4.5 & installed the older version, 4.1 - and right now it works normally. Ive set virus database update to “ask when update is available” and program uptdate to “manual”, so it doesen`t accidentaly update it to newer version.
Maybe (I’m not sure) but this could be a problem…
Not only security but the way things work…
I really suggest the use of a password, even simple if you want.
You can set virus database update to ‘Automatic’ and the new version will never update unless you ask to.
But, really, it’s not a good thing stay with version 4.1 as the 4.5 has a lot of improvements and new features :-[ :-\