WinAd (aka WNAD) is a commercial parasite and something I feel you would
probably rather not have present on your PC.  For details and removal
instructions see
http://us.mcafee.com/virusInfo/default.asp?id=description&virus_k=99236

When did you last check your system for viruses, trojans and other malware
using your preferred
antivirus application using an updated current set of viral reference
signatures?  If not recently then you might first want to check your PC
using one of the online virus checks such as McAfee's online checker
(http://us.mcafee.com/root/mfs/default.asp)  or Trend Micro's Housecall
(http://housecall.antivirus.com) and then update your AV's reference
signature files.  If you haven't yet managed to install an antivirus
application then download and use Stinger
(http://vil.nai.com/vil/stinger/) and perhaps also F-Prot which will allow
you to scan your system from DOS
(http://www.f-prot.com/products/home_use/dos/).

This would be a good time to download yourself a copy of the free Ad-Aware
SE Personal edition  from Lavasoft
(http://www.lavasoftusa.com/software/adaware/) and also SpyBot Search &
Destroy 1.3 (http://www.safer-networking.org/) and use them to check your
system for other commercial parasites remembering that they are only as
good as when you last updated their reference files. I also use a program
called BHODemon (http://www.definitivesolutions.com/bhodemon.htm that
checks for unwanted Browser Help Objects and SpywareBlaster
(http://www.wilderssecurity.net/spywareblaster.html) which can help
prevent some parasites getting a grip on your PC.

Then  there is CWShredder
(http://www.spywareinfo.com/~merijn/files/cwshredder.zip) which is the
best way of getting rid of the many forms of the CoolWebSearch hijacker
details of which can be found at
http://www.spywareinfo.com/~merijn/cwschronicles.html and also
http://www.pestpatrol.com/pestinfo/c/cws.asp..

Finally if you still continue to experience problems download a copy of
HijackThis from (http://www.spywareinfo.com/~merijn/downloads.html).
Create a folder called HJT on C: (not on your desktop nor in your temp
folder) and copy the file you downloaded to that folder. Close as many
applications as you can including all instances of Internet Explorer.
Enable Explorer to see all files and folders (Tools | Folder Options |
View and check "Show hidden files and folders" and uncheck "Hide protected
operating system files") and then run hijackthis.exe and post back the log
to either the HijackThis Forum at http://forums.spywareinfo.com/ or
alternatively http://forum.aumha.org/viewforum.php?f=30 and hopefully this
will enable someone to identify the cause of your problem.

See also: Dealing with Unwanted Malware, Parasites, Toolbars and Search
Engines http://mvps.org/winhelp2002/unwanted.htm and also Browser
Hijacking http://www.spywareinfo.com/articles/hijacked/