Do you have a windows boot floppy disk for your pc? it can be made easily. Go to my comp'> cont' panel> add/remove prog's, and click on the tab marked STARTUP DISK, then follow the instructions. When this is done, write the date and the title for the disk on it, then you put this into your floppy drive, and once you have saved all yhe files you need/want to save, press cont/alt/delete twice, then wait for the pc to start up, then a flashing A:\ prompt will appear when ready. type in FORMAT C: you will then be aked if you are sure click Y for yes(sometimes after the Y you need to press enter. When the format has completed, load in the CD into its drive, remove the floppy and press cont/alt/delete, twice to restart, the pc will then go ahead and reload windows itself, and when complete will eject the cd, put it away safely, and let the computer run until it completes loading windows and makes it operational, and there you are. Keep your boot floppy safe too in case you need it again in the future. Thats how you do it from the floppy drive, on some PC's you can load in the boot CD and restart the PC, and it does it, itself, it may be an idea to try this first.
This explains it better than I can I "borrowed from a forum at click here and wonderfully written by someone called Mark, If he's reading this I'm sure he wont mind me using it here.