Operating system cd is for recovery?

There may be a recovery partition on the hard drive -- in which case if you install afresh from a Microsoft Windows CD you may lose some drivers and utilities normally provided by the laptop maker.

I would contact the laptop manufacturer -- though I'd imagine the issue is explained somewhere in the documents which came with the laptop.

What is the make and model of the laptop? If you have windows 7 then the backup/recovery disks can be created from the control panel/backup.

no its vista, xps/ m1530 ... i used 1 of the CD but it said there is no such a file, .
the actual problem is my drive c had 8 GB free, but suddenly is became full, i though maybe its virus, i removed something but still it has just 100 MB free so i have to recover my pc, but how? i didn't find in the documents...

Why not just delete what ever files caused it to go from 8GB free to 100MB?
You shouldnt need to re-install the whole OS to fix a small problem like this.

Here is the link to you owners Manual
http://support.dell.com/support/edocs/systems/xpsM1530/...
Page 125 gives you the info to restore to factory defaults... You will need to backup all your files first if you chose to do this.