Hello,
without logging off,current user i just want to access different user via Radmin. we are able to do with Remote Desktop(mstsc command) but while using these ,we are getting slow access.
What do you mean "access different user" exactly? To get to the files? Connect to the C$ share of the PC with an account that has local admin rights, you can get to any files that way.
There is no way you can logon a different account to the PC without logging off the current one unless you use "switch user" option from Windows. http://support.microsoft.com/kb/279765