To working with GIT you need at least 1 program on windows systems.

msysgit, can be downloaded on  http://code.google.com/p/msysgit/

tortoisegit, can be downloaded on  http://code.google.com/p/tortoisegit/ (useful for tortoisesvn users)

gitextensions, can be downloaded on  http://code.google.com/p/gitextensions/ (may also be useful, have MSVS2005\2008 integration, shell integration, merge tool)

 http://www.syntevo.com/smartgit/index.html - somebody test it please )

for GNU\Linux users i found very nice tool called cygwin which allow to run UNIX applications on windows console, tools like svn, git, cvs can be just installed as cygwin packages, and after this used from windows console (you need edit system variable PATH for this), you also can install shell like bash, and file manager like mc, and text editor like vim or emacs ), can be downloaded from  http://cygwin.com/

if you have write access, i know only one method to use your rights, you need to set authorization info in url, like this:  http://user:password@sss.chaoslab.ru:81/git/icqjplus.git

basic commands:

git clone  http://sss.chaoslab.ru:81/git/icqjplus.git -- for initial copy of repository

git checkout -b 0.5 origin/0.5 -- for 0.5 branch, or

git checkout -b 0.3 origin/0.3 -- for 0.3 branch

git pull -- for update sources

developer commands:

at first set some info about you:

git config --global user.name "Full Name"

git config --global user.email "somename@…"

than make some changes:

git add -- add file\directory to repo

git commit -a -- commit changes to local repository

git push -- upload your merged repo to server