autocad very slow!!!

Hello, excuse for my english..I'm italian
I have this problem
I have a fileserver Win XP Pro, motherboard MSI P4N SLI, 2 GB RAM, 2
Seagate 250Gb in mirroring, Pentium 4 3Ghz. In this file server there
is a share $D in which there are some configuration of autocad 2005
such as plots, styles etc...so any user who use autocad installed on
any client have the same configurations...in this share there are also
the files DWG used by the users. There are 5 clients on which are
installed Win 2000 Pro, Autocad 2005, Norton AV Corporate. The employee
log on to the client with local administrative privileges, start
autocad, open a file on the share $D on the server Win XP Pro, and
works...
First Problem: when draw a line it takes from 2 to 4 seconds to view
the line!!!
Second problem: when I try to launch a preview it takes from 20 to 40
seconds to preview!!! ...for files not so big..says..medium-small!!!

Note that: the old server was a Win2000 Pro, with only one HD maxtor,
with less RAM, with motherboard less performance...butthis problems
dosent appears!!!
Win Xp Firewall is disabled on to server and on to clients...

I was monitoring the net...and there is a lot of bandwidth...so nothing
bottleneck over the net...

I'm begin crazy for this problem...
Can anyone help me??..Thanks a lot!!!