Not sure if SP2 would be the problem - could be. I can't say for sure because I don't use SP2. I'm patched, firewalled (soft and hard), AV'ed and I use Firefox. The only other thing I could do to be even safer is switch to a Mac.
nmenaker makes a very good point - don't use the most recent version of AC. I'm using 3.30, and not the newer 3.7 or even 3.53. 3.30 works just fine for me.
AC is fantastic. That's absolutely amazing that it knows when you're on your repeater and prints accordingly.
*sigh* Why can't more people see the light and get a Thinkpad...
*deliberate troll alert*