You may want to check this. Quoted from an other post.
Some program (usually spyware\parasite) may have
overridden the URL prefix
setting?
Dealing with Unwanted Spyware, Parasites, Toolbars and
Search Engines
http://mvps.org/winhelp2002/unwanted.htm
Includes: Repair the corrupted or altered (spyware) HTTP
prefixes
See section: Various Registry Fixes
= "RepairDefaultPrefix.reg"
[or - manually]
Open Regedit to the following locations and verify the
below default
entries:
Start | Run (type) "regedit" (no quotes)
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVers
ion\URL\DefaultPre
fix]
@="http://"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVers
ion\URL\Prefixes]
"ftp"="ftp://"
"gopher"="gopher://"
"home"="http://"
"mosaic"="http://"
"www"="http://"
[HKEY_CURRENT_USER\Software\Microsoft\Internet
Explorer\URLSearchHooks]
"{CFBFAE00-17A6-11D0-99CB-00C04FD64497}"=""
Note: edit as needed, *always* Export before editing.
_______________________________________
Source: Mike Burgess
http://www.mvps.org/winhelp2002/