A retail disk/key of Windows can be used on up to 3 installations.
An OEM disk/key of Windows is only good for 1 installation.
You can install both retail versions and OEM versions as many times as you want. The OEM version cannot be transferred to a different computer. You may have to activate the OEM version over the phone if you reinstall it.
This does bring up the issue of George Washington's axe - at what point does your computer become a new computer if you upgrade it? The answer is it's tied to the motherboard, you will probably need to call up Microsoft and ask them nicely to activate it for you if you change it. I don't entirely know how this works since I've never been in this position but it's worth a try.
In any case, you will have no problem reinstalling the OEM version on a laptop since you will probably never replace the motherboard. This is why the license key is generally printed on a sticker and stuck to the bottom of a laptop, just in case you ever need to reformat your laptop.
Ars Technica has a
great article (including the long form of what I just summarized) on reinstalling Windows on laptops. I'd highly recommend anybody purchasing a laptop do this.