I bought a Mac because I thought I would not ever get a virus but my daughter opened a link on fb and the screen went immediately white. I got as far as Command-Option-P-R and now have a screen that gives me four options. Where do I go from here?

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What four options are you seeing? Probably you are in Startup Manager > http://support.apple.com/kb/HT1310 If you see "Macintosh HD", choose it. Then, open System Preferences > Startup Disk, and choose Macintosh HD

What you are seeing is not the result of a virus, or any other kind of malware.

As to what it is, I'm not sure yet. If it will not start up normally, at a minimum the computer's system is corrupt. If you held down command-R (or command-option-R) at startup and got to a screen with four options, one of them should be Disk Utility. Select that one. Do you see your Mac's hard drive in the list of devices? You should see something like this:

Hyperion, in this case, is the name I have given my hard drive. Macintosh HD is the default name.

If you don't see this, describe what you see. If you do, select the hard drive (the item named Hyperion in the example above), then click the Repair Disk button. Once it finishes, did it find any problems, and if so, did it fix them?

This was not caused by clicking that link. Clicking a link cannot cause this kind of problem. That was purely coincidental. As to whether you need anti-virus software, see my Mac Malware Guide.

Sounds like your hard drive had become corrupt, and this likely caused corruption in your system, which resulted in the problems you are currently experiencing. You need to reinstall the system. You can try reinstalling right on top of your current system... just hold down command-R at startup, then select the option to reinstall the system. (This should not interfere with any of your data, but anything is possible when dealing with corruption, so you'd be wise to back everything up first.) You will need to be able to download about 4 GB from Apple's servers to reinstall the system.

If that doesn't help, visiting the Genius Bar would probably be a wise idea. Be sure to make an appointment first.

Buy an external drive, install OSX on it, boot to the external and see if the internal is accessible. The external can be your backup drive in the future. If the internal is no longer accessible, post back.

If you don't have backups, and can't get the system to start up normally, you're in a bit of trouble. You may have lost data already. If your hard drive isn't just corrupt, but is in the process of failing, then nothing short of paying somewhere in the neighborhood of $1,000 to a data recovery service will get that data back if it actually fails.

Csound1 has given you a good option, but if you find that you're not sure exactly how to do what he is recommending, I would advise you to get that machine to the Genius Bar as soon as possible for some assistance.

Once this situation is resolved, you need to make it a high priority to start backing up that machine on a regular basis.