Windows Live Onecare offers several components rolled into one convenient download package. As with many competitors, it is much easier to manage you anti virus, anti adware, firewall, and antispyware by installing one program.
These components are crucial to a properly protected pc. More details on PC security and protection can be found here:
What you need to properly protect your pc
Firewall Basics
Anti Virus Basics
Check if your Anti Virus software is working
Windows Live OneCar offers the following components:
Antivirus & antispyware, Online ID protection, Firewall, Multi-PC management, Printer sharing, and a Backup and restore utility.