|mmm Ed is correct in his suggestion to perhaps check/change boot order to boot from the CD... But first...|
What make/model is the system; and is the disk you have a full version (or Upgrade) Vista disk; or is a Recovery disk (that came with the system? And do yo have personal files, data etc. on the system as is/was that you would like to preserve/save?
If you don't have a full (or Upgrade) version disk then likely you have a Recovery disk. And it may well be that there is a Recovery procedure you can use that doesn't require to boot/use that Recovery disk. All depends on the make/model involved here... On many sytems there is an F-key routine one uses to run a Recovery process (utilising infor on the hard drive.)
This process may have a couple of options - full reset/restore to delivery status; or a partial restore. A full rest/restore "will" erase/lose all data etc. you have on the system at the time... - which means all your personal files etc. will go... So again make/model etc. before you go any further.. It may be possible to recover/save your data first then rebuild the system one way or another...