I use PC Tools "Threatfire" together with Avast 4.8.
Threatfire is solely heuristic if I understand the program correctly (and free.)
No compatability problems on a Vista 32 laptop.
N.B. PC Tools recommend you do not register Threatfire with Windows Security Centre. This is the default loading setting.
My Firewall is Comodo (of course ) They all seem to get on well together.
Hope this is helpful.
Old ground I know but...
heuristic analysis = guess.
By contrast, Threatfire's detection is primarily by analyzing
behaviour, not by fuzzy matching/guessing.
Confusion partly comes from products, even some otherwise reputable ones, using the word heuristic as a buzzword for sophisticated detection method.
E.g. ESET coined the term "Advanced Heuristics" for their behaviour analysis technique.