I have used both Innoculate IT PE and Norton AntiVirus 2001. 
Innoculate IT has good detection rates, and doesn't seem to slow down my system at all and has got good reviews. But I feel that Norton is far superior, because it is a more refined product that has some killer features such as automatic liveupdate (you don't have to remember to updates the virus definition files, it does it automatically!) and the best detection rates of any antivirus package when a scan is run on a particular file or folder. Its Auto-Protect function is good, but slightly behind McAfee's for 'on-the fly' protection, in terms of detection. Norton is slower that Innoculate IT, especially when you try to open up a MS Word document and it scan it before opening it, but it is not too slow. Norton AntiVirus 2001 to me seems to feel quicker then the previous version, and it scans quicker and is less of a resource hog than the old version.
Innoculate IT is good for what it is, but it can't really compete with Norton, but Norton does cost money, but it is worth it.
ANOTHER VOTE FOR NORTON ANTIVIRUS!
NOTE: Stay away from Mcafee Anti Virus! It toally trashed my system when I tried to update it and eventually had to reinstall Windows.:|
Take Care,
UKspace