There are lots of ways to do this I guess. More information on your particular situation would be helpful.
Did you already download Windows 7? If so, did you do it on the laptop, or a different system. If you did it on the laptop you should be able to mount the image file with a program like virtual clone drive. Once the image is mounted, you now have a virtual DVD drive that you can copy the installation files from. You could probably install directly from the mounted image, or a safer bet would be to copy the content of the mounted image to a seperate folder and install from that. If you have a USB drive, you could put it on that and install from that as well. This will work for an "Upgrade" type install.
For a clean install, you are going to need to be more creative. If you post more details, we can help you. There's always a way.
For what it's worth, if the laptop is real old, you might be asking for trouble. Make sure you have a plan to get the system back to XP or whatever it was running. I'm assuming XP because I believe Vista was shipped on DVD.
Are you comfortable with partitioning, etc? Do you have two systems running that are networked? Do you have a USB hard drive?