The problem could be in your system. Are you sure you don't have some malware?
Anyway, the only official place to download UBCD from one of the official mirrors is:http://www.ultimatebootcd.com/download.html
Alternatively, you could search for "ultimate boot cd" in one of many well known software repositories (not official mirrors, but still downloadable).
Both official and unofficial repositories have also torrent downloads for UBCD503.ISO.
In any case (official, unofficial, torrent, whatever), after downloading and before burning to cd, please double check that the ISO image for version 5.0.3 matches this codes:
[MD5 ] 6cf0f4ff7f11ad2d78058541831c4bcc
IMHO, you should check your own system for malware first, because there is no reason to get to any other place than the ones already officially listed, specially if those other sites request money!