Few things drive us more crazy than a one-month-old PC that shipped with pre-installed trial virus protection software.
Once that first 30 days is up, all those trial versions start bugging you with scary "Your computer is unprotected! Click here to purchase a license" messages, meant to terrorize you into dropping cash on ever-upgrading pay-for virus protection.
Our advice? Uninstall 'em, and check out ClamWin.
ClamWin, free antivirus scanner for Windows, is licensed under the GNU GPL (which means it's "free (as in freedom).") ClamWin auto-updates its virus definitons, comes with a scan scheduler and integrates with Windows Explorer and Microsoft Outlook.
Since it's a non-profit, free software project, if ClamWin does help protect your computer, go ahead and send the ClamWin people some money-love.