I have installated Samba on Red Hat 9 - and I am using it as Window$ file server
I have multiple users added in both Samba and Linux OS - with different levels of access - let's say user "a" can see shares 1 and 2; user "b" can see shares 1, 2 and 3
So my problem is with Window$ machines - after connecting and registrating in the file server as user "a" with username and password I am not able to change reasonably my level of access to the file server - with other words I can not change my login name to user "b" without logging off the Windows System and Login on again with differrent username for the windows system/ Win XP and Win 98SE/ - so my question is - is there some other - easy -way to switch between different usernames

Yes and no.

For win98, you have no choice, you must logoff.

For win2000 and winXP, you can use the net use command.
Go to DOS (console) and type: net use /?

This will give you the syntax. You can specify a username and "delete" (disconnect) connections.

If you type: net /?
It will give you a list of other commands and options.