Does the project have a .sln file? Or is it using the older-style .dsp/.dsw project format? (I'm pretty sure VS2003 used .sln, but it's been a while...) You don't need to have the VS.net frameworks available to be able to open the project.
I dug around some of our older example projects, and was able to open (what I think is) a VS 2003 solution on the Mac using 16.1.0.9. Go to the Project > Open Other Workspace > Visual Studio .NET Solution menu item, and browse for the .sln file.
For reference, the top of the .sln file I tested looks like the following. Let me know if your "Format Version" is different than 8.00.
Microsoft Visual Studio Solution File, Format Version 8.00
Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "cpp", "vcpp7.vcproj", "{971A048E-A4BB-431D-9A0D-D58FD4DF09EC}"
ProjectSection(ProjectDependencies) = postProject
EndProjectSection
EndProject
As for the 100% CPU usage, you may have run into a case where one of the SlickEdit helper files is damaged or incompatible with the version of SE you're using. Remove the .vtg (tag database) and .vpwhist (history) files. You may also want to delete the .vpw (workspace) and .vpj (project) files as well, since SE will recreate them as needed when opening a 3rd party project/workspace type.