400 errors mean that the "data" that was sent to the minecraft.net server was corrupted; malformed.
You cannot blame Minecraft for this however, this is your PC. It is likely due to either that Minecraft isn't fully able to send anything because of a firewall, or because there is a virus on your computer (more likely than firewall). Yes loads of viruses do such things. I highly recommend using good antivirus such as AVG (the free version can do good) or Microsoft Forfront (Security Essentials), just to name a few. You can use a free application called "Spybot - Search and Destroy" to detect and attempt to remove any spyware, I would highly recommend that.
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