It's most likely you didn't; we run software which scans all incoming email for viruses. Antivirus software is not infallible(viruses can come out before new "definitions" are released, for example); however, most viruses these days go out of their way to avoid software such as ours by trying to send emails directly, by scanning through the address books or email mailboxes on the system it has infected.
The problems are almost entirely due to massive security vulnerabilities in Microsoft's products(its operating system, email clients, etc.) If you choose to run such a problematic operating system, it is your responsibility to keep it up to date with the security bug fixes which Microsoft releases, AND to run antivirus software.
If you're sick and tired of the constant stream of bugs, crashes, viruses, etc...try using some thing different.