I know enough to blow it up, but not to fix it! Server 2003 disconnecting clients

Hello! I graduated with a degree in Art so it totally makes sense that my boss puts me in charge of the network, but ONLY when we have a problem, right?? We have a contract with an IT company, but they leave a lot to be desired. A LOT.

I have a few issues that I need help with, but I'm just going to start with one! I've been searching Google for hours :confused and I would REALLY APPRECIATE any help!

Yesterday after most of the users had gone home, our whole network (XP clients) disconnected from the server and couldn't synchronize files, couldn't get to shared drives or folders, but could still use the internet just fine. I had to restart the server to get them to connect again. I'm hoping it doesn't happen again today because everyone will be looking to me to fix it, and I haven't a clue why this would happen suddenly (besides that the IT company put 3 new software apps on our computer - they have remote access - over the weekend. All of which look like they essentially do the same thing). None of the other settings have changed that I know of.

Are there any places I should be looking to make sure it doesn't happen again? I can't even begin to think what could have changed with a software install that would cause this. We haven't done anything else on our end, and who knows what the IT company has done?

Thank you!

I'm going back to searching now. I feel like I keep getting close to the answers I need, but they aren't quite right!