I thought it was a driver issue--or at least that's what I remember it being in Windows 2000. I've never had to physically set a switch for multiple monitor support.
Ah hah! A quick check of Microsoft's search engine reveals a list of supported video cards for Windows XP's multiple monitor function:
http://support.microsoft.com/default.as ... US;q307397
Interestingly, I checked on multiple monitor support in Windows 2000 just a moment ago; Microsoft also lists the need for a switch, jumper, or driver for disabling VGA. I've only ever known about cards that used the driver, never cards that actually used a physical switch. That's just weird. Out of curiosity, Speed, which cards had the switches/jumpers?