Snape's ability to spy?

[daisyhead_girl]

No: HPFGUIDX 68717

I hope I'm doing this correctly, this will be my first post.  You'll need to forgive me if 
this has been discussed before.  There are so many posts to sift through.  On to my 
thoughts!

I am questioning Snape's real ability to turn double agent.  His cover was somewhat 
blown in PS/SS at the end.  This is shown as Quirrel/Voldemort talks to Harry in the 
room with the mirror of Erised.  Harry essentially finds out that Snape was not  
cursing his broom during the Quidditch match, that it was Quirrel.  I cannot quote, as 
I haven't seen my copy of PS/SS for years (my hubby keeps loaning it out).   I seem to 
recall something about Snape muttering his stupid counter-curse.  Wouldn't that have 
been a clear sign that Snape was on the wrong side in Voldemort's opinion.  Clearly, 
any DE worth their salt would have loved to see the kid who brought down the Dark 
Lord get a little back.  

So, this brings into question his role in OOP.  What is he really doing and can he do it 
convincingly given the fact that Voldemort really knows that he pretty much saved 
Harry on the Quidditch field already?  This may help support the vampire theory that I 
have seen on several occasions.  

Thanks for a great board!

Vicki