I'm running Windows Vista 64
Just trying to see if there is any correlation, but I'm running Vista 64 too. In the meantime has anyone sent in a Steam Support ticket?
Edit: Try some of this;
Run HL2, exit and then start TF2 (skip this if you don't have it)
Download and install in the tools tab all the Source SDKs
Lower your downloads speeds in tools to a lower value (doesn't matter, Steam download will saturate your bandwidth regardless)
Change your download location
Port forward the following for the Steam client
*Notice* I can't replicate this problem so I don't know if any of this will fix the issue.