What motherboard do you have? I can look in its bios settings and see what might need to be changed to boot from DVD.
try this without CD
start PC 3 times, each time it starts to load windows, restart PC. On 3rd or 4th startup, It should start automatic repair. This probably won't fix problem but once it comes to end it offers 2 choices. Take advanced settings
this loads a blue menu
choose command prompt
type msconfig and press enter
if it opens, choose normal startup and apply. restart PC and it should load normally
if that didn't work, but you can get into the blue menu via the restart thing, go back in and follow same path - advanced/troubleshoot/advanced
choose system restore
if there is a recent date, choose this and it will restart PC and reset settings back to howq they were on thsi date, PC should boot normally.