So you are ready to Clean Reinstall?
Boot the Windows 7 DVD to wipe the HD first with Diskpart Clean Command
accessing DISKPART At PC Startup.
Since Bitlocker is on the 100mb System Reserved partition it should be wiped along with the whole HD and unable to block it unless it has another method to foil Diskpart in a booted DVD which does not need to access the HD at all. We shall see.
If it fails I'd try another method like free Partition Wizard
CD which uses a Linux Boot disk and can wipe the HD from its Disk tab when you click on Disk # to highlight it.