Drag and drop not working in Finder

Hi all

I am working on a Mac Mini running OS X 10.5.8.

I am not able to drag and drop files in Finder and copy and paste operations do not work.

I have read previous posts which state that the SECURITYSESSIONID environment variable being set can cause this problem. The output of env in Terminal on this system indicates that it is indeed set.

My question is how do I unset SECURITYSESSIONID? The posts that I have read indicate that it is set in ~/.MacOSX/environment.plist but I don't have this file on the user account on my system.

How can I remove SECURITYSESSIONID from the environment variables?

Any other thoughts and suggestions about what might be causing this problem would be much appreciated.

Thanks.