I'm left to guess you are trying to connect to a share. That "login "COMPUTER NAME\Guest"" may show if you forgot to create a matching account on both machine. That is, if I log in as bob and try to connect to that other machine, that other machine better have a bob account.
another XP installed computer's connection window doesn't show the other users login windows only one login "COMPUTER NAME\Guest" shows which is also not changeable. Also in both PC the Guest account is disabled.