Why won't my printer work with Windows 7?
The short answer is: drivers.
Think of a driver as a translator; it's the go between that takes commands from your computer, and relays them to the printer in a language it can understand. And like our spoken languages, there are a multitude of printer languages, one or more for each printer manufacturer. And an HP printer won't understand commands written in Dell-speak and Brother won't understand commands written in Xerox. So for each printer you own your computer needs to have installed a unique driver/translator.
Now, to continue the metaphor, the reason your old printer won't work in Windows 7 is that your printer now speaks what your computer considers to be Latin. Or Old English. Sure, there are some translators out there that could make it work, but odds are, unless the driver is specificity designed for the make and model of the old printer, some things are simply going to be lost in translation.
For example, I recently attempted to install an old HP 1029 LaserJet on a new Windows 7 machine, but HP decided not to support the 1029s anymore... ostensibly I was out of luck, but by searching Google I found that I might be able to use the driver for the HP LaserJet3055. And sure enough, it worked, but it kept tossing out errors and occasionally freezing in the middle of a print job. Once more to the metaphor: my printer was speaking in a Southern accent, and the driver I found was from Scotland: it only sort of worked.
So the long and short of it is, when you make the transition to Windows 7 some of your older printers simply won't work. But like anything, their time is drawing nigh. Microsoft is soon going to discontinue support of Windows XP, which will leave it more vulnerable to security breaches, so it's probably time to start budgeting for an office-wide upgrade....including the printers.
The good news is that printers are cheaper than ever before. When you bought that mighty B&W LaserJet 6 years ago it probably cost you several hundred... but these days you can get a color LaserJet fit for a small office for the same or less.
Still hope that your old printer will work? Well, you may be in luck! Search for your old printer on the Windows 7 Compatability Center (http://www.microsoft.com/windows/compatibility/windows-7/en-us/default.aspx);your printer may indeed be supported by the manufacturer. Bear in mind, though, that just because the manufacturer says it has a Windows 7 driver doesn't mean it absolutely will work (the HP Universal driver is notorious for this sort of discrepancy)... each office is unique, and small differences can make the difference between a functioning old printer and, well, let's face it... a door stop.