How to Increase application performance

Hi,
   I developed one HMS application, now its running fine in one or two systems. If i deployed in LAN then we are try to use 8-10 systems. Now the performance is very poor, means slow while loading the application. I developed its in n layer architecture, and its running in general windows7 IIS. So is there any server OS i have to maintain ? and what are the prerequisites are required to increasing the performance. Please help me....