How can you add Ubuntu to a laptop that already has Windows 7 64bit installed? I would consider using a virtual machine, but would need to understand the tradeoffs vs a dual boot.
A virtual machine is a virtual machine. It's slower. Much slower if you don't have Intel-VT in your CPU (and usually laptops don't have that feature).
Besides this possibility, you can install it with Wubi.
(This will also give some drawback in the terms of speed, and you can encounter problems during an upgrade.)
And finally, you can use the built-in partition manager (Click on the Orb, right-click computer, manager, disk management) to resize your partition or you can use Acronis Disk Director suite.
(However, as always, DO a backup.
You could offset the performance loss of running Ubuntu in a virtual PC by running one of the lighter variants.
Either Xubuntu or the newer Lubuntu which both play nicely in a Virtual Box virtual machine with the Virtual Box Guest Additions installed (although you have to do a bit of fiddling to get it to work in the latter)
If you really don't need a "full" Ubuntu this might be a compromise you could live with, particularly if you were just trying Ubuntu out rather than fully committing to using it.
The other answers seem to focus on the drawbacks of virtualization, so I'd like to give you a bit of a different view. I only have used VMWare player, so I'll limit my answer to that.
Using VMware, you can save a lot of time setting up Ubuntu. Just download the VMware ubuntu appliance of your choice (I've always had good experiences with appliances from bagvapp) and you are good to go without any setup. Second major advantage is that transferring files between both operating systems becomes as easy as dragging and dropping the files between two windows.