Did you boot from cdrom? I usually go to bios to set cdrom as boot drive and reboot from there, then if it asks "press a key to boot from cdrom" do so and you should be good to go. You have to press L for removal of partition right? Never had a problem using this method. Only from within windows you get this error i believe as there are system files still present in memory and cannot be accessed or removed.
Anyways try the bios way and see if you can get it working.
If you are trying to do a clean install of Windows, i.e., a full format, before installation, then you don't want to delete the drive. When you boot from the CD at some point will be installing Windows with a warning that there is already an OS on the driver and later one of your options is to do a full or quick format, do the full. Back up all your data because formatting will destroy everything. If you want to Delete something, could be more specific.
not sure on what you are trying to do, clean install, or repair. Make sure you can boot from disk, that is if you are going to do a complete formate and install. If you have a SeaGate harddrive, you can use discwizard. There are also third party programs like partion magic that you can use to formate. good luck. Go to majorgeeks.com for freeware programs that you can use for formatting your disk, this to include Discwizard.