Welcome to SysChat,
Try booting it without the startup disk, if you arrive at the same screen, then try typing:
this will show the listing of the harddrive (seperated one page at a time), if there is a folder called windows and a folder called program files, Windows likely is on the system and you should be able to type win to boot into it.
Start up disks are usually nothing more than a condensed version of dos, thus when you boot the computer with it in the drive, you arrive at the dos prompt C:\>, unfortunayly this prompt will not always allow for use of an OS on the drive.
If nothing is displayed without the boot disk then there is no good OS or no bootable OS on the drive, and you will need to install one. Good Luck.