[Updated 1/17/07 - 12:04am]
Wanted to let everyone know that is was some kind of configuration problem on my machine and you can go ahead and buy Visual Studio Standard after all.
A co-worker suggested getting rid of XNA Game Studio Express on my machine. I did that, but it didn't help. I doubly checked that I had removed all Express editions as well. Then I repaired the VS 2005 Standard install, but that didn't help.
The only thing that helped was installing VS 2005 on a partition that hadn't had it installed before (which happened to be running Vista). I could probably try uninstalling it on my XP partition and blowing away the Visual Studio 8 directory completely, plus settings in my home directory. Oh man, what a pain.
Long story short, it looks like it's time to start rebuilding my Windows install at home :(
For hobbyist programming efforts at home, I finally decided to upgrade from the various Visual Studio 2005 Express Editions. I thought that I could do it with Visual Studio 2005 Standard, which has an upgrade price of around $150. The main thing that inspired this was when I wanted to build an ATL or MFC app with Visual Studio C++ Express, but couldn't because ATL/MFC isn't included with that version. Looking at the product comparison, Standard looked perfect for my needs. It appears to include everything I'd want at home, minus the stuff I use at work (like SQL 2005, Dev Edition).
Well, one itty bitty problem: VS 2005 Standard doesn't let you have multiple projects in a single solution. DUH. So you can't have a C# application project and a related library in the same solution, whether it's C# or C++. That essentially makes it useless for anyone looking to do any kind of serious development unless you want to open and close solutions all day long. Since I only use Professional at work, I wouldn't have ever imagined that Standard would not let me do this. And, of course, no one would know this in advance because Microsoft only lets you download the Professional Edition as a trial.
The only beneficial thing in VS 2005 Standard is that you can load VS plugins and compile for x64, both of which have very limited usefulness. I was able to load the Orcas plugin for designing WPF forms -- something that is inevitably going to work its way down to Express editons anyway if Microsoft has any hope of getting people to write for WPF. And since I have no plans on running x64 soon at home, I don't really need that. But it sure makes a good bullet point on their website.
So here's my advice, don't be fooled by MS marketing and waste your money on VS 2005 Standard: just stick with the Express editions or pay for VS Professional. VS 2005 Standard will probably make managing your projects more confusing than just keeping them in completely different solutions in different Express Editions.