The problem with all software firewalls on XP and before is that they need to patch the Windows kernel to work. Most of the "security suites" also do this for many of their other functions. Why? Because they can. Do you want to work on a machine with a kernel patched by some third-party company, for their own aims? I didn't think so.
The 64-bit version of Vista will allegedly have kernel patch protection (PatchGuard) but this too won't be a real deterrent until they link it to a hardware lock.
