the BIOS in some systems look for a optical CD drive and do not boot correctly a DVD drive, these systems usually have a boot menu which you access by pressing F11 or similar (its a F Key or Function Key) and select the DVD as the first boot.

If you have installed SP2 on a Windows Installation that was allready installed and running prior to SP2 you should be able to access Windows in safe mode by pressing F8 as it is booting and selecting Safe Mode from the option menu...Once you have entered Windows try closing it don and booting it normally, if this does not work you may be able to remove SP2 in safe mode