What are you going to do with your XP machines whose specs aren't up to Windows 7?

I was thinking of formatting the hard drive, reinstalling XP and then using it again, but just as a backup for web surfing, no purchases, no e-mail, no logging into anything. No transfers via usb, as self contained as possible. Would it still be vulnerable to attacks if used this way? If it's on your home network, can it be used to attack other machines on the network?