Well, most of the GDI+ APIs are Not hardware accelerated. Your older programs probably used the original GDI APIs which were usually hardware accelerated. Obviously when hardware gets involved, it usually make things faster.
Exactly which part of your code is running slow? If it would be possible to go back to C# 2005 and benchmark, and then benchmark C# 2008 to see what the difference is between the versions.
Plus try to remember when your done with the app to change the project to "Release" instead of "Debug" which will help 'tighten'/optimize the app.
Also go to the project properties and be sure to check the box that says "Enable Optimizations" which will further use SSE/3DNow and whatnot.
Otherwise, i'm not sure what to tell you, sorry
Jason