I have had a problem before where I get the following message trying to open a solution or project in Visual Studio:
The project location is not trusted: <path to project>
Running the application may result in security exceptions when it attempts to perform actions which require full trust.
Googling the error message turns up lots of hits on how to give UNC paths full trust when opening projects from a network drive. But all of my projects are local. Eventually I discovered that the problem was that the project had been extracted from a downloaded zip file, which Windows XP had "quarantined" in a partial trust zone. The recommend solution is to open the file properties on the zip file and click the "Unblock" button, before unzipping it. Unfortunately, I ran into a situation today where I had already extracted and deleted the zip file. In searching for a way to unblock all of the extracted files, I didn't have much luck, until I found a forum post pointing me to ZoneStripper. ZoneStripper "unblocks" any file or set of files in a directory, which was perfect for my situation. Also included in the readme is a technical explanation of how Windows XP "blocks" downloaded files, and how ZoneStripper fixes the problem, which I found interesting.