Since I had no luck getting WinXP/Visual Studio working in VirtualBox, I decided to try VMWare Player. Long story short, it works! I followed the instructions an a VMWare Player HOWTO at ubuntuforums, and a couple hours later I have a full WinXP VM in which Visual Studio is (apparently) running quite happily. Got TortoiseSVN installed as well. Whee! Networking works just dandy through the NAT'ed virtual ethernet device.
My initial impressions is that VMWare Player gets extremely bogged down by disk I/O. Double clicking a program icon can send it off into space for minutes. It kinda seems to be a one-time hit, though. Maybe I just need more RAM.
Other than the disk I/O problem, responsiveness seems acceptable.
Now I can engage in some Windows development without leaving the cozy womb of Linuxdom.