this is a randomly appearing issue that we at Autodesk have not nailed down the reason for. I have had it happen to me on my laptop and let dev try to find the cause to no avail.
A workaround is to go to Program files > AutoCAD MEP 2010 > and select the AecCB.exe file this is the EXE that is called when you select content browser.
BTW, i reformatted my laptop, spent 2 days reinstalling everything and the issue went away (not saying that is what you should do :)) it may be a corrupt registry or a windows issue that is causing it, but we are not sure. The exe should resolve the problem.
You can create a tool to shell out of ACAD to DOS and start the EXE:
Command: sh
OS Command: "C:\Program Files\AutoCAD MEP 2010\aeccb.exe"
Gregg Stanley [Autodesk]
Gregg Stanley
QA Analyst
AEC
Autodesk, Inc.