Mark,
If you use Spybot, check the Immunize settings. You may have applied the
restriction inadvertently.
If not caused by immunize settings, it was probably caused by an adware,
spyware, parasite program.
See: Dealing with Unwanted Spyware and Parasites: http://mvps.org/winhelp2002/unwanted.htm
Coolwebsearch has been causing this problem lately. Look for CWShredder on
that page.