PC-Cillin, Kaspersky, AVG, McAfee will all probably do the job. I am not a fan of Norton, particularly in an environment where you already have a problem of it running slow. As you can get trial versions, you might like to try more than one?

I would also get SpyBot S&D and AdAware SE.

Remember, update everything and run it in safe mode.....................this is why I am suggesting you pull a machine and check that to see if you do have a malware malware problem, and if so, its extent.

It might even be worth putting a local software firewall on the machine and re-connecting it to see who tries to connect, and on what ports.