Open File Dialog Box Slow to Respond

I rebooted my W2K SBS. After it came back up, everyone began experiencing “slow-to-open-files”. By this I mean, for example, if you are in Excel and want to open a file, the Open dialog box comes up. When you click on the down arrow for Look In:, it takes appx. 20-30 seconds for the drop-down menu to appear. The clients are W2K Pro. It does this for any application. I have rebooted the server since then, as well as the clients. Why did this start happening, and how do I fix it? Thank you.