In Visual C++ 2005, the only supported configuration for managed images is linking them against the DLL version of the CRT (C Runtime Library). This means that if you compile with the /clr switch, you should compile with the /MD in order to link with MSVCRT.LIB (or /MDd for debug versions). Trying to link with /MT to use the multi-threaded static linking version of the CRT (LIBCMT.LIB) is not longer a supported option.

You can find more information on this topic here, in item 12.

P.S: there are no single-threaded variants of the CRT in VS 2005 anymore, by the way.