Every operating system (Windows) needs to be kept up to date. With no updates, your computer is vulnerable to new threats.
Weaknesses discovered in windows after its release make these threats possible.
Fortunately windows is easy to keep up to date by performing windows update automatically, when prompted or manually.

Windows ME, 2000 and XP:
Setting windows update, to automatically download and install updates is recommended for most users.
Right click "My Computer"
Click "Properties"
Click "Automatic Updates"
Select "Automatic (Recommended)"
Under this setting is the day and time to download the updates, every day is recommended and set the time for when you normally have your computer switched on.

To manually update your computer:
Click "Start/All Programs/Windows Update"
Then follow the prompts.