One of the most useful features built into Windows (but not sadly in the home versions) is Remote Desktop. Not to be confused with Remote Assistance, Remote Desktop lets you log into your PC via the internet from wherever you happen to be. It also works on your local LAN, of course, which is useful if you need to access another PC in the house or office without physically sitting at it.
Enabling Remote Desktop takes a single click (it is under Start/All Programs/Accessories) and is easy to do. Though you'll also need to change the firewall settings on your router to allow incoming connections to port 3389, which is the way that RD communicates with the outside world.
It comes as no surprise to learn that hackers are now exploiting remote desktop connections. They have developed automated systems that scour the internet for remote desktop connections and then try to brute-force a login by trying thousands of common combinations of username and password. Once they gain access, they install a web server on the hacked machine in order to host malware and viruses. They then send out millions of spam messages in an attempt to trick people into visiting your compromised computer.
The advantage to hosting their malware on your PC, rather than on a commercial hosting account, is that the infection is much less likely to be detected, and therefore unlikely to be shut down.
If you use Remote Desktop Connection on your computer in order to allow remote access over the internet, you need to do 2 things. Firstly, ensure that your username and password are not easy to guess. Secondly, go to the Local Security Policy on your computer and set a password threshold and a password lockout. Values of 5 passwords and a 30 minute lockout are sensible options. This means that, if anyone does try to guess your remote desktop credentials, they'll be locked out for 30 minutes after each 5 incorrect guesses.