I've seen this myself and never figured it out. ADB is quite fragile in my experience and if you google, you'll see lots of folk with the same problem.
I'm assuming from your post that you are skilled. Apologies if not and come back for simpler help
Some things to check:
1. If you are firewalled, check that ADB (or ADB-WINDOWS if that's what you're using) is added as an exception
2. If on Vista/7, right click ADB.EXE and select Properties->Compatibility. Select "Run this program as administrator"
3. Try this
What does it say at this point?
4. Uninstall the driver and use my tool to reinstall by manually browsing to the drivers you downloaded when it all worked.
The latest Android SDK has updated ADB and the drivers. Come to think of it, I haven't had an ADB problem since installing it so maybe it's a fixed problem?
Edited by simonta, 19 April 2011 - 08:08 PM.