I have a couple clients with security systems, typically I see they run a dedicated PC with at least 256mb RAM, and the system does nothing else.
Video is very memory intensive. If you aren't doing e-mail on that computer, I would consider uninstalling Norton Antivirus. That's going to affect things big time. At the very least, run MSCONFIG, go to startup tab, and uncheck all background apps that you don't need. Hit OK and reboot.
In addition to the steps already mentioned, I also suggest you periodically delete and recreate the storage area allocated by the video capture software (it should have one), and be careful defragmenting your system. That software may not like another application trying to defragment the space it takes up, it could wreak havoc on your filesystem. If you must defrag, make sure that storage area is deleted, then defrag, then recreate it.
Matt J.
Please always take the time to backup any and all data before performing any actions suggested for ANY problem, regardless of how minor a change it might seem. Also test the backup to make sure it is intact.