My 404 erro page has been hijacked

I am running IE 5 on Win NT and while looking at a US Govt site got infected by what i imagine is an activeX script from www.orbitexplorer.com.

They have replaced my 404 error page in IE5 and every time I mistype a domain name I am asked if I want to install gator.com product and get their 404 error page.

I have tried for hours to find how to remove this but to no avail.

Can anyone please tell me exactly how I get rid of whatever is making this happen.

Many thanks
Bruce