Try the Windows repair utility.
Turn on the computer and when the first screen comes up, hit whatever key it asks for to enter the BIOS.
In the BIOS find the BOOT order option (it may have a slightly different name). You should see a list of items such as (Hard Disk, CD, USB, etc).
Rearrange the list so that CD is the first device the motherboard will look for an OS in.
Save and Exit the BIOS. Restart the computer and insert the Windows Install Disk.
On the screen a Windows setup/homepage should appear. Find the link that says "repair". Follow the instructions in there to perform a repair of the Windows operating system.