I have NIS 2007 too (version is 10.2.30 - please check your version by opening About Norton Internet Security) and it does not slow down the PC.
What is your LiveUpdate version. The current version is Version 18.104.22.168. If you don't have that version, get it from:
See if anything wrong with your NIS 2007 by running a scan using their autofix tool (online)
Sometimes installation of any application is corrupted which caused trouble, if any of the above does not help.. try this:
If you still have the NIS 2007 installer or your NIS CD, proceed. If none, get the installer from Symantec by downloading the setup file for NIS 2007:
Download Norton Removal Tool from http://service1.symantec.com/SUPPORT/tsgeninfo.nsf/docid/2005033108162039
Allow it to remove Norton. It will also remove LiveUpdate.
Do not connect to the internet. Install NIS 2007 again.
Run LiveUpdate to get the program updates. Reboot when done.
Run LiveUpdate again to get the other updates. Reboot again.
See how it goes.
Note: If you have other Symantec products other than NIS 2007, do not run the above tool because it will remove them or affect the other Norton product.
I just upgraded NAV 2006 with Norton Internet Security 2007 and had a lot of trouble getting LiveUpdate to run, but after about 10 hours/2days with Norton's tech help got it to run; BUT!! I notice my computer is running EXTREMELY slow compared to before NIS 2007. Has anyone else experienced this?