Joey,
Although sometimes you can "get away" without uninstalling the previous installation, it's usually not the best for antivirus programs. In fact, McAfee recommends uninstalling all previous antivirus programs, including it's own, prior to installing a new one.
With Mcafee, I go the extra mile after the standard installation. I clean out the registry using the link below. It may take a little longer but it helps get a clean installation.:
Searching the Registry for McAfee VirusScan Files
http://ts.mcafeehelp.com/faq3.asp?docid=68373
Another suggestion, after removing the old one, clean out the Temporary Internet Files folder, delete all .tmp files from the computer, then run Scandisk/Chksk and Disk Defrag to opimize the hard drive. After that, restart the computer, then install the new program.
Hope this helps.
Grif