For the last few weeks, I get a notice from BitDefender Antivirus every time I launch Windows, which reports 14 instances of BehavesLike:Win32.ExplorerHijack infecting file c:/windows/system32/svchost.exe=>:ext.exe. "svchost.exe" is a system file, so deleting it manually is not an option. I've searched BitDefender's knowledge base & forum and other resources and have yet to find a solution to the problem. I have read, however, that ext.exe is a malevolent extension and has to be removed, though how to do that eludes me.
I am using an up-to-date version of BitDefender Antivirus 2009 on a Windows XP SP3 system. I am connected to the internet through a firewalled router. Other computers in my home network (also using BitDefender) do not seem to be affected.
Here are screen caps of my Antivirus scan results:


Here is the infected file (password: infected):
/applications/core/interface/file/attachment.php?id=4395" data-fileid="4395" rel="">svchost.rar