PG2 is solid piece of software. "(...) <Urden> pg2 isn't being actively developed anymore apparently but peerblock is forked from pg2 codebase and is being actively developed (...)" "(...) <Jessay> Peerblock is better for vista/ultimate. Phrosty is no longer going to develop PG2 infact he suggests peerblock on the site itself. So basically PG is slowly going to become pb"
PG2 and PB use the same lists format. In next year format for list is going to change. So PG2 could be not be compatible as not developed. But "(...) <fakhir> I-Blocklist will always offer lists in the P2P (PeerGuardian2) format. Updates will be available for PG2 for the foreseeable future. (...)"
So IMHO Vista and later users can use PeerBlock, however it is very very freshy. Win2k till Vista can stay with PG2 for "the foreseeable future", and can have hope PB will be compatible.
Your is choice. I will keep using PG2 as long as possible, because I trust it and it is tested&trusted IMHO.
btw PeerBlock ver 1 has already appeared as I know.
About PeerBlock read here
http://www.peerblock.com/docs/faq.