The license for your OS is built into ROM so you don't need an original disk to install it legally on your machine. If your laptop came with
Windows 7 Professional 64bit, there are a couple of possibilities:
1. If you didnt already wipe the drive, it may have a recovery partition you can use to re-install the system and drivers as it was when you got it.
2. Contact Dell tech support. If your machine isn't too old, they may still have recovery disks that they may sell for ~ $25 (guessing).
If you installed your own copy of Win 7 Pro, or your recovery partiton doesn't work or even exist, you could also borrow any OEM copy of the same vintage from a friend. Assuming your copy is legit, you already own the license. All you need is the installation medium (disk or flash drive) so your key should work to install it.
While you're at it, burn a your own copy, or at least an image file of it in case you need to repair or re-install it again.
Good luck.