A - What username and password do you need to put in?
If its your Windows credentials, that is the login screensaver doing that (has nothing to do with XAMMP)
B - if you are getting error messages that 'XAMPP' has caused a problem (error - that is the XAMPP control panel - not Apache or MySQL. You should look in the Windows System Event Log to see the details.
C - Was MySQL ever installed before or is it currently installed in a standalone mode? If so, that could be causing a problem. Realisitcally, there can only be one MySQL service on a server.
D - on that old a system its nearly inpossible to tell what might be wrong, so I would begin by using the Control Panel to un-install the XAMPP installer or in the Start Menu, locate XAMPP and use the Unintall XAMPP option.
Now that I know you have Windows 2000 - I'll point out you don't have the same problem as teh orginal poster, so you should have started a new Topic. Anyone else with your problem would NOT look in this topic, because its about VISTA!