You have Gold Edition of Sam? If yes, it's already patched to version 1.07. With this you can go back to 1.05 version. There is different players and servers.
Install:
Unrar the archive (mods,data,bin) to your Serious Sam SE main folder, and you're ready to start play 1.05 version.
(To have both versions copy/paste your sam folder to two locations, and install this patch to other)
+ From 1.07 version+
+ No full server bug when server is empty + Smoother gameplay + Better ping
