Virus Protection

I consult SOHO networks and run into this question very frequently. Small networks with an average of 10 users need virus protection just like the rest of us. Running Windows Server 2000 with the file server enabled requires a file scanning solutionfor viruses. Installing a package like Norton Virus or McAfee does not work properly and in some cases does not even install properly (depending on the options of the server). What is the best solution for this scenario without having to pay for a corporate virus package?
Also, why do virus scanners cause Windows 2000 profesional to run poorly with higher crash potential?