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:
OS Command: "C:\Program Files\AutoCAD MEP 2010\aeccb.exe"
I discovered that after I opened Content Brower from windows it then opens fine from within MEP10. Or at least it did immediately after I closed Content browser, I didn't get a chance to try it again yesterday (after restarting the computer)
My Next issue is: There in hardly any content in it!!!!! I have only the "My Tools" and "Content" (don't remember the name for sure, I'm at home now... but it had links to other people's online stuff) There was no MEP or Arch tools at all... Not even the Standard tool palettes.