Do you have children? Do you have other users that rely on your computer?

If yes to either, then you should create multiple accounts to decrease the chance of viruses/spyware from malicious software.  By default, a machine with one user automatically loads that account as the "Administrator".  An Administrator account allows all software to be installed despite it being "good" or "bad" and the computer's registry to be changed without any notification; the combination of these two can be very detrimental for a computer since it allows malicious software to have free-reign over your computer.

For your personal account, you can leave it as the Administrator and it's recommended that you password protect it.  For all additional users, you should make them Limited accounts and they can password protect themselves later if they choose.  A Limited account cannot modify the registry or install most software programs without Administrator rights.  Even though this is not a 100% way to block viruses/spyware, it will drastically decrease the chances.

The steps are pretty simple!

For Windows XP:

1. Click the "Start" button
2. Click "Control Panel" on the right column
3. Click "User Accounts"
4. From there you can "Change an account" to password protect your current account and "Create a new account" to add more user accounts with Limited access.

For Windows Vista:

1. Click the "Start" button
2. Click "Control Panel" on the right column
3. Click "User Accounts"
4. From there you can "Change your Windows password" to password protect your current account and then click "Add or remove user accounts" followed by "Manage another account" to add more user accounts as a Standard user (limited access).

In addition to keeping your virus protection software updated, this method will greatly help in avoiding viruses/spyware.