Hopefully I have rectified the problem - I have uninstalled Threatfire and have run three full scans with Norton Internet Security 2007 with no problems. It seems Threatfire was taking up to much resources for NIS 2007 and Threatfire to work concurrently.

In relation to having one Spyware and Anti Virus programs operating at one time on a PC, with the removal of Threatfire I have Spyware Doctor 5 & NIS 2007 as my Spyware and Anti Virus programs. Ad Aware 2007 & AVG Anti-Rootkit do not have realtime protection and are only used to scan.

You can have more than one as long as they aren't Realtime, but even scanning you can't use more than one.