Out of the free antivirus program you can find online, I think Avira Antivir is the best one. It may not be the best in terms of detection rate (I think Kasperky wins out in this one, but I feel that its performance is very bad), but dare I say, for 99.9% of the time, it detects all viruses that have ever grace my system, and it doesn't bog down system performance noticeably.

The only annoyance is this big bad nag screen being displayed after every signature update. I have found a way to disable it. Here goes...

.: Disable the Avira AntiVir avnotify nag screen :.

Windows 2000:

1. Click Start, and then click Run
2. In the Open box, type gpedit.msc, and then click OK
3. Expand User Configuration, expand Administrative Templates, and then expand System
4. In the right pane, double-click Don't run specified Windows applications
5. Click Enabled, and then click Show
6. Click Add, and then type the executable file name of the program (avnotify.exe) that you want to restrict users from running.
7. Click OK, click OK, and then click OK (NOTE: If domain-level policy settings are defined, they may override this local policy setting.)
8. Quit Group Policy Object Editor
9. Restart the computer

Windows 2000/XP: (2000 & XP Pro do not require you to boot into safe mode)

1. Boot into Safe Mode (tap F8 repeatedly after you restart the computer)
2. Log in using the Administrator account
3. Go to C:\Program Files\Avira\AntiVir PersonalEdition Classic\avnotify.exe
4. Right-click avnotify.exe-> properties-> security-> advanced
5. Under the Permissions tab click on SYSTEM under Permission entries:
6. Edit-> Traverse Folder / Execute File-> deny-> ok ->apply-> yes -> ok-> ok
7. Reboot the computer into Normal Mode (start-> shutdown-> restart)

Windows XP Pro & Vista Business/Ultimate/Enterprise

1. Start-> Control Panel
2. Administrative tools-> Local security policy
3. Click on Software Restriction Policy-> Action (at the top)-> create new restriction policies. If there are pre-existing restriction policy, this step is not applicable.
4. Right-click additional rules (on the right side)-> new path rule
5. Click Browse and navigate to C:\Program Files\(Avira)\AntiVir PersonalEdition Classic\ and double-click avnotify.exe
6. Set the security level to Disallowed-> apply-> ok

Windows Vista Home Premium

1. Go to C:\Program Files\Avira\AntiVir PersonalEdition Classic\avnotify.exe
2. Right-click avnotify.exe-> properties-> security->
3. Under the group or username SYSTEM click edit
4. Put a checkmark under the DENY column for "read and execute"

.: Disable the splash screen :.

1. Open regedit and navigate to [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
2. In the right pane double-click avgnt and add /nosplash at the end of the path (ex. /min /nosplash)

quoted from http://www.elitekiller.com/files/disable_antivir_nag.htm