What example are you trying to build - a little research shows that bufferoverflowu.lib(and other related files bufferoverflow*.lib) were in previous SDKs to support compiler building before and after Microsoft added the ability for the compiler to create a buffer overflow check (starting with the / GS option in VS 2003 - I think).
SDK, Windows , , SDK, , bufferoverflow*.lib . , C, .
Windows SDK v7.0 VS 2005 SP1 , C - bufferoverflow*.lib .