what are you trying and where is it failing?
usual steps
1) backup your data OFF your pc (more important data multiple backups)
2) boot up enter bios and set boot order (CDROM/DVD then HDD)
3) put in windows disc (or whatever you want/have) and boot from cd/dvd
4) follow prompts and setup windows
5) once finished install drivers and activate windows
6) transfer data back
if your windows wont boot at present TEST YOUR HARDWARE THERES A REASON WHY ITS NOT BOOTING
if its not booting from the cd/dvd recheck the boot order, perhaps your CD/DVD drive is faulty
if your struggling perhaps get a shop to do it for you