Wendy, your PC is infected by a variant of the WinAntivirusPro malware.
You won't be able to install any new security software until that is dealt with.
You can use one of the free online scanners to try and remove the infection:
http://www.eset.com/onlinescan/
http://www.ewido.net/en/onlinescan/ <<== this one should work
http://onecare.live.com/site/en-us/default.htm
You'll also find a slew of threads in the antivirus and antispyware topic folders here from people who encountered this malware. Although most are sent to support, if they are using OneCare, there are links to solutions for removal of the infection.
I'll close with the standard response regarding an infection reported here:
If you are using Windows Live OneCare and you have been infected, but OneCare did not detect or cannot remove the malware, please contact support to report this and for help with removal.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If you are not using Windows Live OneCare, you are off topic for this forum. This is not a general forum for viruses, spyware, or Windows Help. For help with spyware issues, you may want to try the forums here:
http://aumha.net/ For help with virus removal, contact the maker of your Antivirus program.
If you are in North America, you can call 866-727-2338 for help with virus and spyware infections. See http://www.microsoft.com/protect/support/default.mspx for details. For international information, see your local subsidiary Support site.
-steve
