Here's the best one! I'd add more but there's a max of 15 options...
Windows Xp
Windows 2000
Windows Me
Windows 98
Windows 95
Microsoft OS/2
DOS (any version)
Mac OS 9.1
Mac OS 10.1
BeOS
OpenBeOS
Unix (any version)
Linux (any version)
BSD
Any other OS
Here's the best one! I'd add more but there's a max of 15 options...