Other programs that use Com ports include Palm Pilots and other PDA's. So would the Uniden UASD program, for the 996 or any other scanner, or WINxx for a GRE scanner. Make sure all of them are closed out.
Open up the Device Manager & make sure the port is active.
Depending on your version of Windows, it may be accessible by <right> clicking on My Computer on the Desktop (for versions prior to WinXP) & secting "Properties.
In XP, you may find My Computer on your Start Menu. <Right> click & select properties there, or go to the Control Panel & open up "System".
Under the "Hardware Tab" is a button for "Device Manager"
Opening the Device Manager will show you a list of hardware devices on your pc (drives, modem, controllers).
You'll see an item labeled as Ports.
See what port is listed under COM. Most commonly, you'll have COM1. If you have a modem (dial up) as well, you might also see another, often COM3.
Make sure ther is no yellow exclamation mark or an "X" in the port branch. That indicates a malfunction.
If Device Manager identifies the available Com port as COM1, make sure that you set ARC996 for COM1.Also be sure the baud rate is set the same on the scanner & the software.
Note that if you've been able to connect with the Uniden software, the problem is likely in the ARC996 software settings, or another program (UASD or WINxx if you use one of it's versions) not releasing the port.
Using the demo version should not be your probelm. Butel's demo's generally allow full funtionality, just with a limited number of downloads to the scanner. You might download the latest beta & see if there is any improvement. It will operate as a demo since you have not purchased, & entered, the software key.