Could be that you now have too much stuff on there !
As a general rule, I wouldn't run more than one anti virus product on the same machine.
Without knowing the spec of the machine or exactly what's installed, difficult to give specifics, however as a set of general guidelines:
1. Are the basics right ? Enough hard disk space remaining ? Enough memory ? Extra memory is cheap and the best way to boost a flagging machine. Lack of it means the machine is constantly writing data to disk. Although this shouldn't affect startup too much.
2. Give your PC a clean out, uninstall as many additional programs as you can using the Add/Remove programs feature in Control Panel. Make sure you have the disks if you want to reinstall ! If in doubt about what a particular program does, don't uninstall it. Reboot afterwards.
3. Once you've done this, run some system tools such as scandisk and defrag to tidy up the disk. Reboot afterwards
4. Reinstall a good, up to date antivirus product and firewall. We use Sophos which I find to be extremely good at detection, but requires a bit of poking around in the configuration to get it to automatically remove the virus for you. Norman produces a product called Internet Control, which is anti virus and software firewall combined. Only install one at a time, if it doesn't find anything, feel free to try another but uninstall the first and reboot before installing the next one.
NAV is a good product but in my experience does impose quite a system overhead.