Windows 7 CPU usage high for Explorer.exe and DWM.exe

The explorer problem sounds like a misbehaving shell extension.
Shell extensions are Windows Explorer addons that add additional functionality. CD burners, compression utilities, some backup systems, and other programs will add shell extensions to integrate with Windows Explorer.

However, a poorly-written shell extension can slow down your machine. Since they are explorer addons, they would show up as CPU usage in Explorer.exe.

Use this utility to disable every non-Microsoft extension that you don't actually use.

You can also run Process Explorer and look at the call stacks of the misbehaving processes to get a general idea of what they're doing.