The 2 culprits are usually Skype and IIS (Windows Internet Information Server).
To find the ports in use double click on xampp-portcheck.exe
in the xampp folder, in Vista you need to run as Administrator by right clicking on the exe file.
Perhaps configure Vista so you are a Super Administrator:http://www.winvistaclub.com/t12.html
Turn off UAC to use XAMPP features:
See Goofy2k's other post:viewtopic.php?f=16&t=32915&start=0