When i try it in IE it directly logs in to 'user' account. And if it is from other machines it directly goes to guest account. May be it is using windows account to login. But when used firefox it does displayed login box but even though I gave a valid user name and password it goes to guest login. How to solve this problem and login to different users

The default installation of SplendidCRM is configured to use Windows Authentication, which gets you single-sign-on.  You can disable Windows Authentication and enable Anonymous Authentication in IIS, and then SplendidCRM will prompt you for an internal, as in SplendidCRM managed, login and password.

Firefox does not support the same automatic Windows Authentication that IE does, which is why Firefox presents you with the typical popup login dialog.  Windows Authentication should still work in Firefox, but you will need to provide your Windows login and password.  Again, if you disable Windows Authentication and enable Anonymous Authentication in IIS, then Firefox will display the SplendidCRM login page just as IE does.