It's one of the following.
1. Boot order in the BIOS.
2. Dirty media or lens.
3. Bad CD/DVD drive.
This is why I carry a bootable OS on memory stick around.
When turned on - all I get is a continual spinning of the microsoft logo. If I hit F12 during turn-on - I get 2 choices - either START WINDOWS NORMALLY - Where I get what I just described.
STARTUP REPAIR - When i choose that - It (startup repair) starts - and also runs continually (I had it run for nine hours yesterday).
I can't run any discs to try to reload anything - because the computer will continue to run either the spinning microsoft logo - or the startup repair.