IF you don't have access to the install CD, you'll want to do two things.
First is to create a system repair disc
, as outlined in the URL. This will function as 'the next best thing'.
Second is to create a system image backup
. It's not described in the video, but you just click on the 'create system image' link and follow the instructions, rather than going through the normal backup procedure.
If you -only- have a C drive, you will need to either get another hard drive and install it, or repartition your C drive into multiple drives (Easier in Windows 7, though still not recommended). You can also back a system image up straight to DVD or Blu-Ray, but this isn't as reliable - better to burn the disc with the known good backup, instead.