Problem: Replaced NT4 server with 2003 SBS

[I_am_Vash]

ok this is the network setup:
a)1 server: running NT4 domains being used
Intel PII 233, 256mb RAM, 4gig IDE hard drive
b)6 clients: 5 running XP pro and 1 running win98
All P4s with 256mb Ram and 20gig hrad drives, one (win98) AMD 1800+ with 256mb ram and 20gig hard drive
c)24 port 10baseT network hub
d)internet access through dial up on server

So here is the problem:
we replaced the server with a P4 2.4c, 1024mb ram, two 120gig hard drives in mirror raid mode, and 2003 Small Business Sever. We used the same domain name (that is what the administration wanted), unpluged the old server and replaced it with the new one. Now some times the new server will not allow people to log on the first time; it sometimes will tell the users that the domain could not be found, then you cancel the login and try to do it again and it will let them in on the second try or after a reboot. Next it sometimes will disconnect the user from the server without notice, you could be working along then try to access the server and it will tell you the network resource can not be found, so you have to reboot to fix the connection. Another problem is when deleting files from the server it will say that you can't delet that file because it is being used, and when you check with the other clients no one is using that file, rebooting does not help. Finnally the network runs really slooooow.

At first I though it was the hub, they wouldn't let me change it so I went to my next option and unpluged the new server and put the old one back...... none of the problems appered, so I unpluged the old one and put the new server back and all the problems started up again. I am confused and I don't know where the problem is, can any one help me? Thanks