Bonk.. which version of windows are you running and which video card?
If XP and NVidia, then
Turn off Anti-Virus
Install OP
Patch to v2552 then use the OP Server Directory fix (or just get the OP Enhancement Package v5.1)
if using the OP Enhancement Package v5.1 Check the WonServerSetup.gf file and put the " " marks around the Directory servers, remove the Stasis entry (invalid while Firesoul's server is offline), this was a temporary fix for XP and D2 due to the Taldren Firewall Detection (removal of the " off the server entries) this issue you fixed for XP and Vista..
Open OP directory and find StarfleetOP.exe, right click, copy, paste shortcut on to desktop.
right click shortcut and select properties
Click on compatibility tab
put checkmark into Disable Desktop Composition
Click OK.
Make sure you have latest edition of Diret X
I also recommend using NVidia's Beta Drivers located at :
http://www.nvidia.com/object/winxp_174.74.html (Windows XP, XP Pro, XP Media Center Edition [x86 or 32 bit])
Disable firewalls and keep Anti-Virus disabled ..
try to connect.
Launch first time.. wait for firewall message, then ALT-F4 out of game.. immediately re-launch OP and try to connect to Dynaverse..
You should be able to log in now.
Repeat steps for launching after you turn your AV back on.. see if it will allow you to connect.. if not, your AV is blocking you.
If you can connect, then re-enable your Firewall, if unable to connect, then firewall not configured correctly or port forwarding is incorrect and may need to be re-installed/refreshed
This usually works for 99% of XP players to get 100% functionality on D2