IBM-DOS 3.3 on an IBM PS/2 Model 50
Then IBM-DOS 4.0 on an IBM PS/1 (I forget what model, but it was a 286 with 1MB RAM)
Upgraded to MS-DOS 6.22 and Windows 3.1 (a tight fit on a 30MB hard disk)
Then on to new machines with Windows 98 SE, Windows 2000 Pro, Windows XP Pro, and now Windows XP x64 edition. Simultaneously at various times, various distributions of Linux (through 2.2, 2.4, and 2.6 kernels), Windows 2000 Server, and Windows 2003 Server.
Then IBM-DOS 4.0 on an IBM PS/1 (I forget what model, but it was a 286 with 1MB RAM)
Upgraded to MS-DOS 6.22 and Windows 3.1 (a tight fit on a 30MB hard disk)
Then on to new machines with Windows 98 SE, Windows 2000 Pro, Windows XP Pro, and now Windows XP x64 edition. Simultaneously at various times, various distributions of Linux (through 2.2, 2.4, and 2.6 kernels), Windows 2000 Server, and Windows 2003 Server.