Simply put... Remember the second season? Trying to "change the locks" to keep Angel out? Hiding the crucifix behind the curtains? When Buffy asks, her response? "Ira Rosenberg's only daughter nailing crosses to the wall? I have to sneak over to Xander's to watch A Charlie Brown Christmas. It's worth it, though, to watch him to the Snoopy dance..." (or words to that effect).
Willow / Alyson Hannigan - BuffyGuide.com