Choosing the laptop's operating system might be tricky. Windows Vista and 7 have a new audio system. It took a while for manufacturers to catch up with compatible hardware and software. Until recently, Windows XP was the best choice for recording.
Now you can find components that work with Windows 7. But older hardware may not be compatible. Also, using more than 4GB of RAM requires a 64-bit version of Windows. Not all hardware has 64-bit drivers. Check audio hardware compatibility carefully before you buy.
Of course, you don't have to go with Windows. A MacBook Pro has the hardware requirements necessary for this task. As recording computers, Macs are generally very stable and responsive. Just be sure you buy OSX-compatible recording hardware and software.
The laptop should be for music purposes only. Get it to the point where it works and leave it alone. You want to be sure it will always work when you need. Keep it off the Internet as much as possible. Don't even let it connect to a network except your own. You want viruses to stay away.