It totally matters. Just ask any Mac or Linux diehard. Personally, I use all three of the primary operating systems: Windows XP, Mac OSX and Linux (Ubuntu in this case). I have separate uses for each and can't imagine getting rid of any of them.
Note: I have heard that Linux is getting better for this stuff but windows is cheep enough that I don't really care.
No connection to them other than I'm a happy customer ;)
The one Dell running Windows has 1) an HP scanjet 4470c that isn't supported in OS X or SANE and 2) MS Money with 10 years of data. GNU Cash is on my todo list and I found an epson scanner on craigslist for $40 that I'm going to get tomorrow. Once I resolve those two issues I will happily abandon my Creative Suite 2 and never use Windows again.
Meanwhile, people at Starbucks ask me how to get on the internet and while I help them with their HPAcerVaioToshibaGateway $500 special, I advise them to use Ubuntu instead of Vista. I give them the link to Ubuntu and I know they won't use it. One guy was so happy he bought me a coffee, even though I'm sure he will never follow that link. They use what ships with the computer.