Vista - Trying to install antivirus, but told I already have one running?

First of all, sorry in advance for the ignorant question. I tried to
download a free antivirus software (AVG) and it went through the process
until at the end it told me I already had an antivirus running and having
multiples running might cause a problem. Thus, the installation did not
complete. However, I don’t believe I have any other antivirus on the
computer currently. I did have CA Security Suite, but recently uninstalled
it. I have since double-checked and there is no indication that CA remains
on my computer or that its un-installation did not take, nor is there any
indication that I may have another antivirus currently in place. Does anyone
have thoughts on why AVG thinks I might already have something else
installed? Is there a way to bypass this problem and install it anyway, or
in the alternative is there a way to find out what AVG thinks I have?
Thanks, and have a nice day.