XP to Mac

I have and ibook OSX 10.3.5 and win xp sp1.  im just 
hooking the two computers up through their ethernet 
cards.  i use the mac and can log on the windows computer 
without a problem, but when i try to log onto the mac it 
comes up with a uname and pword prompt.  i have tried 
almost every combination of username and pwords for the 
mac and even the windows machine but it won't log on. i 
had the same problem about 6 months ago but it finally 
logged on and i have no idea how.

thanks for the help