I am having trouble with installing postgresql. I installed it before and it was working good then i uninstalled it completely from my system because of  some problem.Now I am not able to install it again.  "user name already exist " or "internal account lookup failure: no mapping between account names and security ID was done'

If you don't want to delete your postgres user (It is tricky to delete user in Windows Vista) then during installation you can use postgres as your Service Manager Account and provide correct password of postgres user. It should work. Becuase, I installed One Click Installer and then I unstalled it becuase instruction says that pgInstaller is better than One Click Installer. I faced same problem but I could not delect postgres user! So, I just provide postgres as username and admin as password. So far it is working good...