Windows 7 seems to have most drivers installed as default. My Epson 6100 laser printer is an example of that.
My HP Officejet 6500 is like that You can get a huge "full featured" driver, or a simpler "basic scan and print" one.
I used to download the basic one on XP (the full featured one never seemed to be stable), but since Windows 7 if you add it as a network printer you get the built in driver which is fine. No scanning, but if you plug in a USB cable you get TWAIN drivers installed.
I actually tried the basic driver to get network scanning but it failed halfway through install. Doing a quick Google search it seems like that is quite common and yet there is no fix for it. In fact if you read through the 'fixes' people have suggested, most of them end up with "add the printer from the control panel", which really means "use the drivers built in to Windows".
So the built in drivers - which are perfectly stable, the only problem is I need to plug in a USB cable to scan - are the only ones worth using. And they're based on the venerable UNIDRV, a technology which has been around since Windows 3.1. Basically UNIDRV lets the you create a text file which defines the escape sequences for a printer. No wonder they don't support scanning! in fact one of the GPD files mentions
*CodePage: 1252 *% Windows 3.1 US (ANSI) code page.
So it seems like HP actually maintain a "Full" driver, a "Basic" driver and the config files for UNIDRV which end up baked into Windows. You have to wonder why they bother, particularly as the "Basic" and "Full" drivers seem to be such a nightmare to support and even get working.