Well, I would suggest one of the many Unices available out there.
They are free, no need to worry about viruses or spyware, and they run much faster than Windows.
Here are some of the more mainstream Linux distros, if you are curious.
Yoper:
http://www.yoper.com
Fedora Core:
http://fedora.redhat.com/
Debian:
http://www.debian.org/
And the *BSD based...
FreeBSD:
http://www.freebsd.org
Macintosh OS X:
http://www.apple.com
OpenBSD:
http://www.openbsd.org