Are you running several partitions on your hard drive, or is it one continious partition? The vast majority of new laptops (Vista onwards) now come with a seperate recovery partition which contains the entire OS to enable you to re-install the operating system should any problems arise.
If this is the case and you do have this recovery/OS partition, then you have two options. You can delete the partition (only if you have media backups of it) or you can leave it be. Strictly speaking it will do absolutely no harm where it is, but will allow you to restore the previous operating system at a later date. That said, my Dell Vostro laptop (my 2nd one. lol) uses XP Professional and I dont plan to upgrade that just yet. lol.
On another note, if you haven't got proper media backups (on disc) of your laptops default operating system I strongly advise you make them immediately. You can usually do this by finding the manufacturer's name as a menu inside the start menu in windows. If you have the proper discs, and you don't have a recovery partition then dont worry. lol.
If you have no critical data (e.g. you are happy to lose everything) on your hard drive, you can do the following to completely remove all data:
1. Insert your W7 install DVD.
2. Boot your laptop and start the W7 install.
3. Select custom install, not upgrade (which should be disabled from a boot install anyway leaving you only one option to install W7)
4. when you get to hard drive section, you need to select the partition you want to format, then in the below boxes you need to click on the format option. This will delete and format the partition with absolutely no data at all.
The install is typical in every respect prior to choosing your default install drive, and after selecting, and formatting the hard drive and continuing the installation.
I'm now running dual boot W7 Ultimate x86 and x64 on all my computers, so if you get stuck I'll help you out.