No problems with avast and no exclusions on my systems, other than exclude the c:\windows\temp\_avast_ folder (where avast unpacks/sends tmp files to be scanned).
I have never had to do that on my XP Pro system.
E5200 2.5GHZ, 4GB RAM, 320GB HD, Windows 7 Home Premium 64bit, avast! V9.0 Free, IE10
P4 2.8GHZ, 1.5GB RAM, 40GB HD, XP Pro SP3 32bit, avast! V9.0 Free, Google Chrome
with hpHosts, MVPS HOSTS files, SpeedFan, WinPatrol PLUS