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This advice covers three types of home page locking - locking (by ISPs and computer manufacturers), locking by "protective" software, and hijacking (by malware or viruses)
Some computer manufacturers and suppliers of internet 
					access set IE to their choice of home page and lock this 
					setting via the registry. Hijackers use the same trick. The 
					locking is done using registry settings as per the 
					following:
					
					Home Page Setting Changes Unexpectedly, or You Cannot Change 
					Your Home Page Setting (Q320159)
					
					http://support.microsoft.com/default.aspx?scid=kb;EN-US;q320159 
					
					
					Specific registry settings affected are:
					
					[HKEY_CURRENT_USER\Software\Policies\Microsoft\Internet 
					Explorer\Control Panel] - DWORD "HomePage"=dword:00000001 
					(grays out the whole section)
					
					[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer] 
					- DWORD "NoSetHomePage"=dword:00000001
					
					[HKEY_CURRENT_USER\Software\Policies\Microsoft\Internet 
					Explorer\Restrictions] - DWORD "NoSetHomePage"=dword:00000001
Check your protective software (especially antispyware 
					and antivirus).  Spybot Search and Destroy, for 
					example has a feature that will lock your home 
					page.  Other products that may lock your home page 
					including Ad-aware's Ad-Watch, SpywareBlaster, SpySweeper, 
					Norton AntiVirus, McAfee VirusScan and Antispyware, and both 
					versions of Zone Alarm.
 
If your computer home page is set to about:blank against 
					your wishes, or any other page, you have a malware problem. 
					For advice on fighting malware, check out the link below:
					
					http://inetexplorer.mvps.org/tshoot.html
					
					If you are using Spybot S&D, check your 'Immunize' 
					settings which may be locking your home page.
First, update to the latest version of Internet Explorer 
					available for your operating system. Most sites that try to 
					hijack your home page will now trigger a 'do you want to do 
					this' warning message that lets you stop the process. The 
					sneaky background activex downloads that are often used by 
					hijack sites to install spyware will also trigger a 'do you 
					want to do this' install window.
					
					Please see 
					this link for advice on how to prevent browser 
					hijackings. 
					
					Make sure that your antivirus is completely up to date and 
					scan all files on your computer's hard drive(s) regularly.
					
					
					There is a little shareware programme that can help stop 
					your home page and search engine settings from being 
					changed.  The programme makes it much easier to take 
					back control of your home page and search engine settings - 
					mind you, there are other utilities, such as Spybot, that 
					provide more comprehensive protection - this utility is 
					purely to protect your home page:
					
					http://www.pjwalczak.com/spguard/