Spyware replaces search engine URL

Some of the viruses or spyware replaced the search engine that IE 6 calls when wrong URL is entered in the address field. Even if virus and spyware is removed, I still have problems. Also, it is normally possible in IE to enter URL like techrepublic.com and IE adds http://. Now it is not working, I alwasy get error messages. How to restore it to the original settings? Where it is defined? How to prevent that from being replaced?

Gregory