I would recommend using a firewall if you don't use one already. ZoneAlarm
is a good free one, but it is only free for non-comercial use I believe. A firewall allows you to regulate incoming and outgoing Internet activity.
If you use your PC for a business, and coworkers, employees, or partners also have access to it, then the firewall settings are going to be a bit more indepth, I am not that familiar with those requirements, but Im sure if you need some advice for that, someone here at BC can help.
Other than a firewall, you should have a base Anti-Virus program, and some Spyware/Adware/Malware protection. If you send a lot of messages that are important, a good encryption program wouldn't hurt either. Here are a few recommendations to get you started.NOTE: all are free for non-commercial use.AVG Anti-Virus Spybot Search & DestroyAdaware SE PersonelSpyware BlasterSpyware GaurdaSquared Windows Defender (Beta2)
Hope this helps,