To boot from a dvd disc you will have to enter the bios of the computer, F1,F2 or the Del key on first power up of the system at the post splash screen.
In the bios there is a section that will say system boot, or Boot order of devices.
You need to change the Dvd drive to number one in the boot list and any hard drives after it in the list set them to 2.3.4 ect depending on how many you have. save the settings in the bios and then exit and restart the system.
It should say after it press any key to boot form Cd/dvd drive. To load the install setup of the Os from the dvd disc.
You can try a quicker fix but it may, not work. If you have a windows Iso xp boot disk you can use that, once it has loaded to set up, in the repair options you can drop to a dos command box.
Once there you can type two commands, fixboot, and fixmbr.
Once typed, it will ask if you are sure you want to preform the task say yes.
Do both, then re boot the system, in some cases it will repair the boot record and the master boot record section, allowing windows to load, even if it is windows 7.
Not sure on windows 8 not tested it myself but it does work with windows 7 in some cases.
I am giving you this solution because it can fix the booting problem into the os. to allow you to change the settings you turned off in Msconfig for the service loading, and start up programs on boot into windows. it will allow you to re set them back to what they were before you messed around with them.
And it is a quicker way, than having to re-install the whole Os over again.