Sometimes you're at a building who proxies their internet connection and you need to get a copy of some code from sourceforge.net.
The Subversion FAQ has great info on this http://subversion.tigris.org/faq.html#proxy
The specific file you want is %APPDATA%\Subversion\servers
Edit the [global] section
# is the comment character. Remove it to enable the following sections
http-proxy-host # the IP address or hostname of the proxy (don't include http or https)
http-proxy-port # the port the proxy service is hosted on
http-proxy-username # username (sometimes you need the netBiosDomain\username)
http-proxy-password # password
http-timeout # in seconds
http-auth-types # authentication protocols.