Windows PC Security isn't simple, but here is a simple explanation of the threat.
The bad buys have changed their tactics in the arena of viruses/malware/spyware over the years. No longer is it something that you notice right away as they delete your files and mess with your PC. Today when the bad guys get control of your PC they try hard not to be noticed. Because control of your computer is worth money to them and they can get control of thousands of computers a day with automated programs called malware, virus, whatever label you want to call the bad guy programs.
Once they've got control they have programs that comb through the PC to find logins and passwords or just have a program wait for you to login to a site and steal your passwords as you type them in.
It's important to realize this is not something they do personally, PC by PC, they run this on hundreds of computers at once. Probably while they are out partying, spending the money made from renting ten of thousands of compromised PCs to other bad guys.
They need to continue compromising other PC's as people get smarter and defensive software gets better. So they will use your mother or sisters Yahoo email account to send a link, which installs malware for Windows PCs, to everyone in her address book. And the cycle continues...
Not taking steps to protect your computer leaves your friends and relatives exposed to the bad guys.
You might not consider your computer access significant, but your identity online can be used to hurt those you know.