connection string problem

Question

friends in my application when i use following connection string  while doing set up project in  windows forms it running successfully on each and every PC

Data Source=.\\SQLEXPRESS;AttachDbFilename=" +Application.StartupPath + "\\dm\\hm.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True

but same connection string when i use in class library which is a data acces layer it is not working please suggest me comparitive connection string