InfinityLimited

Many programs do not use the built in XP proxy configuration. Firefox, Dropbox, Google Translate Client to name a few.
Moving between networks is a pain, and I had previously tried to automate the task of changing the proxy settings, with mixed results.
Enter Squid.
This article explains how to configure it to connect to an parent proxy that requires authentication.
Make sure you change the http_port parameter to only bind to 127.0.0.1, unless you’re doing something sneaky with your iPad, which cannot use the parent proxy….

Update:
A few more usefull tidbits:

• Install Squid as a service with the -i command line option
• Start and Stop with the net start|stop commands
• You should modify the ACLs from those in the article linked above. Here are mine:

acl localhost src 127.0.0.1/255.255.255.255
acl all src 0.0.0.0/0.0.0.0
acl INTERNAL src 172.28.0.0/255.255.252.0
acl NAUGHTY_STUFF dstdom_regex →
  -i "etc/domain_blacklist"

http_access deny NAUGHTY_STUFF

http_access allow manager localhost
http_access deny manager

http_access allow localhost
# http_access allow all
http_access deny all