I've owned a Mac since the original (1984) and a PC since 1983. I own both now - a Mac Mini and Dell laptop w/ XP on it. I use them both daily. They network to each other just fine via SAMBA/SMB. I like my Mac better.
You can use MSN Messenger on the Mac. You can play .mov and .mpeg and .avi movies in QT player. To play .wmv files you'll need a plugin from MS called Flip4Mac which isn't the best.
You can download Remote Desktop from MS to log into a Windows account from your Mac.
If you buy a Mac with the Intel processor (most new ones except the PowerMac) you can install XP on a second partition and dual boot using BootCamp free from Apple.
You can also run Windows virtually using various programs such as CrossOver and VirtualPC.
Addendum:
WOW, obviously there are people commenting on Macs who have like haven't used one in like a decade. The two Macs that ship with a mouse (PowerMac and iMac, the others are laptops and the Mini) ship with a 4 button roller-ball mouse (not just a 1D wheel).
Also, since most all UNIX software is usually recompiled for OS X, well, that pretty much blows the Windows has more software argument out of the water. Sure, Access isn't available on the Mac but MySQL is ;-)
Hum, and I take a Mac laptop to places where I have to hook it up to a VGA projector all the time. Gee, just plug it in and it automatically switches to extended display mode where as my Dell Insperion you have to press a button and change the monitor settings - what a joke.