I have just uninstalled NortonAV and replaced it with AVG. When I re-started I found tthat my WindowsXP Security Centre had stopped and I cannot get it to GO again despite following instructions (re-start again,open Control Panel, click Security Centre)

Can you help?Try downloading and running the RNAV tool from Symantec.com to completely REMOVE all of the Norton Antivirus program:

http://service1.symantec.com/SUPPORT/tsgeninfo.nsf/pfdocs/2005033108162039Thanks. But already did this.It is possible the disabling of the Windows Security Center is due to an infection by a computer virus.  Did you uninstall NAV because is was not FUNCTIONING properly?  If so, this might also point to a viral infection of the system.  

Try a couple of these free online virus scans:

http://www.google.com/search?hl=en&q=free+online+virus+scan&btnG=Google+Search

If your system is not infected, you might have to do a repair installation of Windows XP (which will REQUIRE a Microsoft Windows XP installation CD):

http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx

If all you have is a system restoration CD, you can use it to restore the system to the state it was in when it came out of the box.  You might lose your DOCUMENTS, pictures and music, and might have to install some of your programs after the system restoration.  

If you do not have a system restoration CD, you might be able to get one by contacting the manufacturer:

• Dell support (USA): 888-560-8324

• Gateway support (USA): 800-846-2301

• HP & Compaq support (USA): 800-474-6836

• IBM support (USA): 800-IBM-4YOU