Ubuntu Software Center in Ubuntu 10.10 gives error message waiting for apt-get to exit, and will not install new software.

While trying to use the Ubuntu Software Center software that came with Ubuntu 10.10, it worked perfectly until just lately, Now when I try to install packages from internet sources or cd sources I get an error when checking the install progress indicator: waiting for apt-get to exit. As if there is another copy of Ubuntu Software Center running, but there is not but one running. It resfuses to install new packages evevn from the trusted Ubuntu sites. What other inforamtion do you need to help with this question? Maybe my sources.list file from ????? what directory? Thanks in advance for your help,

-Sid
South town, Atlanta GA.
Thanks!