You're welcome iriegirl. The unregistered person said it well, about IP addresses being "recycled". This would more likely be your case if you have recently obtained Cable Modem access.
There are some things to note about Cable Modem access though. Because your computer will be connected to the Internet 24/7 (or at least, it is supposed to be), you become a bit easier target to malicious users.
A software based firewall is a good start, but I'd recommend adding a hardware firewall as well. Such firewalls are usually built into "routers" (see
http://www.linksys.com, which provides these products and good information about it too). They're fairly inexpensive (around $50). In addition to that, your computer will have a different type of access to the Internet, rather than direct access to the Internet.
Even though you may not be using a home network, you can still use the router for just one PC. It'll give you some added security, and when you decide to do add an additional PC in your home, you can use both those PCs to access the Internet with a bit more ease.
Obviously, even hardware firewalls can be comprimised, but the thing is to make it "harder", not "easier" to get into your system.