any programs--especially pricey ones--require you to enter a product key or registration number when you reinstall them. If you lose that key, you lose the ability to reinstall it. Typically the keys are printed on the box that the program ships in, on a disc case or sleeve, or on some loose piece of documentation, so they're pretty easy to lose. (I always write the key directly on the installation CD, using a permanent marker, before installing the program for the first time.)

If you no longer have an installed copy but you've registered the program with the manufacturer, a quick call to customer service should deliver a product key without too much hassle. If you haven't registered it, expect a rockier road; when asked, Microsoft, Adobe, and Symantec representatives all said that customers could obtain a registration key for an unregistered product, but first they would have to provide verifiable proof of purchase--such as the original CD or an invoice--and jump though various other hoops as well.