In addition to the above suggestions take a look at Indiginus Solid State Symphony. Pretty good for a synth sound and it doesn't cost much.

I have been using a combination of several libraries ranging from Philharmonik 2 to some of the new strings in HALionSOnic 3. I love the Sonivox (Sonic Implants) Strings which I have used in their SoundFont format and they sound great.
Recently I have picked up Albion ONE and Output Analog Strings which I have not yet had a chance to install as I am trying to figure our my VST/Sample storage management.

Hmm, I used the included SI Strings Section for a while, sounds as good as any I ever used and lots of ways to tweak it,,.. but then I decided,, I actually hate strings on pop music! Reminds me of the 70's..  So now I use different cool sounding Poly synths / Pads that don't even try and sound like real strings. Poly synths are considered vintage ya know!  Forget those stings, been way overdone, start playing with those synths 
 
My Fav is the Ultra Analog Session try Bank 4- Pads-  patch 11-Poly

My favorite so far is LASS, although they are not cheap or light on computer resources. They sometimes take some effort to program depending on what you want to do, but I've found them quite versatile and a solid choice for most needs. If you have money to invest and want to buy one library, that would be my top pick to check out. Listen to comparisons against Hollywood Scoring Strings, though, and see if you like that better. I haven't tried Hollywood Scoring Strings, which I've been told is very good.

I've also used Cinestrings Solo and East/West Symphonic Orchestra. Symphonic Orchestra is okay, but it's a pretty old library and just doesn't have much life to my ear compared to other libraries out there. Plus you don't get solo instruments out of it.
 

The Ketron SD4 - http://www.ajamsonic.com/sd4module.html

Session Strings Pro..

I use East West Symphonic Orchestra almost entirely for strings.  Where you run into problems with inexpensive VSTs are in the articulations.  The attack, release, etc are not right, they sound really fake and unnatural. East West also has  Keyswitches along with other settings.  You can switch between articulations while playing that will make them sound real. 

Vienna is good - if you get the right collection.  East-West probably makes more sense for this particular use.
I did not like ANY of the Garritan String Sounds.

For rock, I find myself always going back to the old Roland U110/U220/CM64 string sound as it kind of lies between a mellotron and a "real" string sound. It's also not too thick so it fits in the mix well.
 
For more realistic strings, I use either the SI String Section or the Sonivox Orchestral Companion Strings.
 
M.

The free included Cakewalk Studio Instruments Strings is all that you probably need for pop/rock backing tracks.  Next step up for cheap might be the Sonivox Orchestral Strings, available for $29.99 until April 2nd.
 
Garritan Personal Orchestra 5 is now 50% off at $74, until April 11th.
 
If money is no object, and you seek perfection or illumination, then the sky is the limit after that!